Geometric Design Manual

Section 7

SECTION 7 TABLE OF CONTENTS SECTION 7: CROSS SECTION ELEMENTS ........................................................1 7.1 Introduction ........................................................................................................1 7.2 Headroom and Lateral Clearance........................................................................1 Figure 7-1: Headroom and lateral clearance.................................1
Table 7-1: Additions to headroom at crests ......................................................................1 Table 7-2: Minimum lateral clearances for traffic lanes, foot- and cycleways ..............2

Figure 7-2: Headrooms and clearances for traffic signs................2 7.3 Road and Lane Width .........................................................................................3
Table 7-3: Width of edge strip and edge line....................................................................3

7.4 Shoulders ............................................................................................................3 7.5 Normal Cross Fall ..............................................................................................4 7.6 Side Slope and Back Slopes ...............................................................................5 Figure 7-3: Roadside regions........................................................5
Table 7-4: Slope Ratio Table (Vertical to Horizontal ratio).............................................7

7.7 Drainage Channels .............................................................................................7 7.8 Clear Zone ..........................................................................................................8 Figure 7-4: Heights and clearances...............................................9
Table 7-5: Clear zone widths............................................................................................. 10

Figure 7-5: Example how to calculate clear zones......................10 Figure 7-6: Clear zone correction factor for bends.....................10 7.9 Side roads and culverts......................................................................................11 7.10 Right-of-Way ..................................................................................................11 Figure 7-7: Right of way of road.................................................11 7.11 Four-Lane and Divided Roads .......................................................................12 7.12 Single Lane Roads .........................................................................................12 7.13 Median ............................................................................................................13 Figure 7-8: Median designs at speed limit 80 and 100 km/h......13 7.14 Cross-section over bridges and culverts..........................................................14 Figure 7-9: Segregated footway on bridges................................14 7.15 Footways and cycleways................................................................................14 7.15.1 Footways and cycleways in rural areas................................................15 Figure 7-10: Separate foot and cycleway on rural roads.............15 7.15.2 Footways and cycleways in built-up areas........................................15 Figure 7-11: Raised, kerbed footway in urban areas...................15 Figure 7-12: Footway/cycleway on physically separated shoulders.......................................................................................16 7.16 Service roads...................................................................................................16 Figure 7-13: Service roads..........................................................16 7.17 Typical Cross Sections ..................................................................................16
LIST OF TABLES
Table 7-1: Table 7-2: Table 7-3: Additions to headroom at crests......7. Error: Reference source not found Minimum lateral clearances for traffic lanes, foot- and cycleways .............................................................. 7. Error: Reference source not found Width of edge strip and edge line....7. Error: Reference source not found

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Section 7

Slope Ratio Table (Vertical to Horizontal ratio)..........7. Error: Reference source not found Clear zone widths ............................... 7. Error: Reference source not found

LIST OF FIGURES
Figure 7-1: Headroom and lateral clearance......7. Error: Reference source not found Figure 7-2: Headrooms and clearances for traffic signs. . .7. Error: Reference source not found Figure 7-3: Roadside regions............................... 7. Error: Reference source not found Figure 7-4: Heights and clearances..................... 7. Error: Reference source not found Figure 7-5: Example how to calculate clear zones......7. Error: Reference source not found Figure 7-6: Clear zone correction factor for bends.....7. Error: Reference source not found Figure 7-7: Right of way of road........................... 7. Error: Reference source not found Figure 7-8: Median designs at speed limit 80 and 100 km/h.........7. Error: Reference source not found Figure 7-9: Segregated footway on bridges.......7. Error: Reference source not found Figure 7-10: Separate foot and cycleway on rural roads....7. Error: Reference source not found Figure 7-11: Raised, kerbed footway in urban areas....7. Error: Reference source not found Figure 7-12: Footway/cycleway on physically separated shoulders 7. Error: Reference source not found Figure 7-13: Service roads ...................................... 7. Error: Reference source not found Figure 7-14: Typical normal cross section and values of various cross section elements for Road Design Classes Bitumen Ib, II, III, Gravel A, B, and C................................................................................................. 7. 17 Figure 7-15: Typical normal cross section and values of various cross section elements for Design Class Gravel A, B and C roads........................... 7. 18 Figure 7-16: Typical town cross section and values of various cross section elements for Design Classes Paved Ib, II, and II roads....................... 7. 19 Figure 7-17: Typical Normal Cross Section for Design Class Paved Ia Road ........7. 20 Figure 7-18: Typical Town Cross Section for Design Class Paved Ia Road ...........7. 20

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Section 7

SECTION 7: CROSS SECTION ELEMENTS 7.1 INTRODUCTION

A road cross section will normally consist of the road way, carriageway, shoulders, curbs, drainage features, median and earthwork profiles. Road way is the portion of the road, consisting of the shoulders, the carriageways and the median. Carriageway is the portion of the road used for the movement of the vehicles exclusive of shoulders. Earthwork profiles are the side and back slopes of the road cross section.

7.2 HEADROOM AND LATERAL CLEARANCE

Headroom is the required height to allow traffic to pass safely under objects restricting the height. It should normally be provided over the full width of the carriageway.

h ea d ro o m

Foot and c y c lew a y

h ea d ro o m tra ffic la n e s

la te ra l c le a ra n c e
Figure 7-1: Headroom and lateral clearance
In determining the

The maximum legal height for a vehicle in Uganda is 4.0m. headroom standard, allowance must be made for: The road surface being raised during pavement overlay work The possibility of an overbridge collapsing if hit by a vehicle The need to allow for occasional oversized vehicles.

The recommended headroom under bridge structures should be 5.0 m on class A, B and C roads and 4.5m on lower road classes. The headroom should be 6m under highpower cables and 5 m under low-power cables. The minimum headroom over footways and cycleways should be 2.5m. An addition to the normal headroom is needed at crests with radii below 700m see table 7-1.

Table 7-1: Additions to headroom at crests

Crest radius (m) 100 Addition (m) 0.12 200 0.06 300 0.04 400 0.03 500-700 0.02

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Lateral clearance, see figure 7-1, is the minimum permitted distance between the edge of the traffic lane, the footway/path or cycle way and the nearest fixed object. Fixed objects must not be so close as to discourage the driver from making full use of the traffic lane. The recommended lateral clearances are given in table 7.2.

Table 7-2: Minimum lateral clearances for traffic lanes, foot- and cycleways
Speed limit 30 height lower than 0.2 m height higher than 0.2 m guardrail roofs etc 0.00 0.30 0.30 1.00 50 0.00 0.60 0.60 1.00 80 0.25 1.50 0.60 1.00 100 0.25 2.00 0.60 1.00 0.00 0.30 0.00 0.50 0.00 0.15 0.00 0.50 Footways and cycleways 1) Footpaths1)

Impacting object

1) including hard shoulders for walking and cycling

Minimum headrooms and clearances for traffic signs are given in the MOWHC Traffic Signs Manual see Figure 7-2.

A B C D E F

Minimum (mm) Desirable (mm) 600 1000 300 600 300 600 1000 1800 2100 2100 1) 5000 5200 1) maximum allowed vehicle height 4.00 m F Could be decreased to 4.5 m on category C and D roads

Figure 7-2:

Headrooms and clearances for traffic signs

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7.3 ROAD AND LANE WIDTH

Road width should be minimised so as to reduce the costs of construction and maintenance whilst being sufficient to carry the traffic loading efficiently and safely. Lane width and condition of the road surface have the greatest influence and effect on safety and comfort of driving than other features of road. In cases of paved road when we say lane width it is the net width excluding width of edge strip, edge line and center line markings. Lane widths of 2.80, for Design Class Paved III and Gravel B, to 3.65, for Design Class Paved Ia, are used. Higher design speed requires wider lane width than relatively lower design speeds. The capacity of a road is affected by the width of the lane. Narrow lanes force drivers to operate their vehicles closer to each other laterally than they would normally desire. Restricted clearances have the same effect. In capacity sense the effective width of travelled way is reduced when adjacent obstructions such as retaining walls, headwall of structures, and parked cars restrict the lateral clearance. When continuous two-way right-turn lanes are provided, a lane width of 3.0 to 3.6 m could be provided. For road class Paved Ia a 3.6 m right turn lane is considered from the 5.0 m median width. A lateral/edge strip is a portion of the road way contiguous with the traveled way for recovery of errant vehicles, for lateral support of base and surface course. It also permits drivers meeting or passing other vehicles to drive on the very edge of the roadway without leaving the surface. The width of the edge strip depends on the design speed.

Table 7-3: Width of edge strip and edge line

Design Speed [km/h] 120 100 80 60 <60 width of edge strip [m] 0.75 0.45 0.45 0.20 edge line [m] 0.15 0.15 0.10 0.10 -

For access roads with low volumes of traffic design class E roads, single lane operation is adequate as there will be only a small probability of vehicles meeting, and the few passing manoeuvres can be undertaken at a much reduced speeds using either passing places or shoulders. As long as sight distances are adequate for safe stopping, these manoeuvres can be performed without hazard, and the overall loss in efficiency brought about by the reduced speeds will be small as only a few such manoeuvres will be involved. It is not cost-effective to widen the running surface in such circumstances and a basic width of 4.0 m will normally suffice. Single lane road is dealt with and further discussed in Section 7.12.

7.4 SHOULDERS
Shoulder is the portion of the roadway contiguous to with the carriageway for the under listed purposes or functions. Shoulders are recommended for all Design Classes and may be paved when the carriageway is paved. This has a number of advantages. It will prevent and protect the carriageway pavement from edge failure and ravelling. It will accommodate a very heavy pedestrians and other non-motorized traffic that would otherwise, especially during inclement weather, use the road way and interrupt vehicular traffic. It will provide a better surface for vehicles parking and vehicles requiring immediate repair. The width of shoulder varies from 1.2 for Design Class 7.3 Ministry of Works, Housing and Communications

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Gravel C up to 2.5 m for Design Class Paved Ia. Shoulders are intended to perform the following purposes: To provide additional maneuvering space on roads of low classification and traffic flows; To provide parking space at least partly off the carriage for vehicles which are broken down; Safety margin to enable drivers to recover control; To enable non-motorized traffic (pedestrian and cyclist) to travel with minimum encroachment on the carriageway; To provide lateral support of pavement structures; and, To act as barrier for moisture egression.

It is recommended that all shoulders of paved roads be paved, though exceptions may be made for very low volume roads. Regardless of the width, a shoulder should be continuous. Paved shoulders are a safety feature and they also contribute to structural integrity and lower maintenance costs. A continuous paved shoulder provides an area for bicyclists to operate without obstructing faster moving motor vehicle traffic. Shoulders intended for use by pedestrian and cyclists must be at least 1.5 m wide. Where there is a lot of pedestrian and cycle traffic the shoulder may be widened to 2.0 metres, but it would be much safer to provide a separate footway/cycle way. Furthermore, shoulders on structures should have the same width as those on the rest section of the road way. The narrowing or absence of shoulders, especially on structures, may cause serious operating and safety problems. Long, high-cost structures usually warrant detailed special studies to determine feasible dimensions. Reduced width shoulders may be considered in rare justifiable cases. Use of different surfacing material can make the shoulder more visible, and this helps drivers to avoid straying onto it by mistake. However, it is not recommended to make the surface of the shoulder rougher than that of the traffic lane, because this will discourage pedestrians and cyclists from using it. All shoulders should be sloped sufficiently to rapidly drain surface water but not to the extent that vehicular use would be restricted. Paved shoulders normally have same slope as the traffic lanes and should be sloped from 3 to 6 percent, gravel and crushedrock shoulders from 4 to 6 percent and turf shoulders up to 8 percent. Much smaller clearances will sometimes be necessary at specific locations such as on bridges, although a minimum of 1.0m will remain desirable. Minimum overall widths in such circumstances should be sufficient to allow the passage of traffic without an unacceptable reduction in speed, which will depend on the length of the reduced width section and levels of motorized and non-motorized traffic flow. Separate facilities should be provided for pedestrians where possible.

7.5 NORMAL CROSS FALL

Two-lane and wider undivided carriageway on tangent sections or on very flat curves have a crown or a high point in the middle and slope down ward towards both edges. These are also called as cross fall or camber. Cross fall or camber slope should be sufficient to provide adequate surface drainage whilst not being so great as to make steering difficult. Steeper cross slopes requires a conscious effort in steering and increases susceptibility to lateral skidding when vehicles brake on wet pavements and even on dry pavements when stops are made under emergency conditions. The ability of a surface to shed water varies with its smoothness and integrity. On unpaved roads, the minimum acceptable value of cross-fall should be related to the need to carry surface water away from the pavement surface effectively, with the maximum value above which erosion of material starts to become a problem.

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The normal cross fall for paved carriageway on tangent sections and on very flat curves with larger radius, shall be 2.5 percent maximum. Higher values up to 3 percent could be used in areas of intense rainfall in order to facilitate roadway drainage, even though undesirable from the point of view of operation. On high type two-lane carriageway the crown slope of as low as 2 percent is accepted for all other conditions. The normal cross fall for unpaved roads should be 4 percent.

7.6 SIDE SLOPE AND BACK SLOPES

Side-slopes should be designed to insure the stability of the roadway and to provide a reasonable opportunity for recovery of an out-of-control vehicle. The design of side slopes is of great importance for traffic safety, as this can determine whether drivers that run off the road are able to recover control. The shallower the slope, the safer it will be. And the transition from the shoulder to the foreslope must be smooth enough to prevent the vehicle becoming airborne. A safe transition is also needed between the foreslope and the back slope so as to avoid causing the vehicle to rollover. Three regions of the roadside are important when evaluating the safety aspects: the top of the slope (hinge point), the side slope, and the toe of the slope (intersection of the fore slope with level ground or with a back slope, forming a ditch). Figure 7-1 illustrates these three regions.

(1) Hing Point

Carriage Way

(2) Side slope

(S) Back slope

(4) Ditch bottom 1 H1 Shoulder Variable width (3) Toe of slope H2 1

Figure 7-3:

Roadside regions

The hinge point contributes to loss of steering control because the vehicle becomes airborne in crossing this point. The side slope region is important in the design of high slopes where a driver could attempt a recovery maneuver or reduce speed before impacting the ditch area. Safe transition between the side and back slopes should be provided as there is a chance of a vehicle reaching the ditch is somewhat on the higher side. Rounding at the hinge point though not necessary from a vehicular rollover can 7.5 Ministry of Works, Housing and Communications

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significantly reduce the hazard potential. Similarly, rounding at the toe of the slope is also beneficial. In general rounded slopes reduce the chances of an errant vehicle becoming airborne, there by reducing the hazard of encroachment and affording the driver more control over the vehicles. Embankment or fill slopes parallel to the flow of traffic may be defined as recoverable, non-recoverable, or critical. Recoverable slopes include all embankment slopes, 1:4 (1 vertical to 4 horizontal) or flatter. Motorists, who encroach on recoverable slopes, can generally stop their vehicles or slow them enough to return to the roadway safely. Fixed obstacles, such as culvert head walls, should not extend above the embankment within the clear zone distance. A non-recoverable slope is defined as one which is traversable, but from which most motorists will be unable to stop or to return to the roadway easily. Typically, vehicles on such slopes typically can be expected to reach the bottom. Embankments between 1:3 and 1:4 generally fall into this category. Since a high percentage of encroaching vehicles will reach the toe of these slopes, the clear zone distance extends beyond the slope, and a clear runout area at the base is desirable. A critical slope is one on which a vehicle is likely to overturn. Slopes steeper than 1:3 generally fall into this category. The selection of a side slope and back slope is dependent on safety considerations, depth/height of cut or fill, characteristic of soil or natural ground material, and economic considerations. Further, the guideline in this section may be most applicable to new construction or major reconstruction. On maintenance and rehabilitation projects, the primary emphasis is placed on the roadway itself. It may not be cost-effective or practical because of environmental impacts or limited right-of-way to bring these projects into full compliance with the side slope recommendations provided in this guide. Table 7-4 indicates the side slope ratios recommended for use in the design according to the height of fill and cut, and the type of material. However, this table should be used as a guide only, particularly as concerns applicable standards in rock cuts, where a controlling influence is cost. When the embankment (fill) height is greater than about 1.0m, the 1:4 foreslopes recommended in the table becomes uneconomic. This is because a large amount of fill material will be needed and the structure will extend over a large area thus increasing land acquisition costs. In these circumstances the foreslope is best determined by the natural angle of repose and erodibility of the material (often 1:1.5). Where steep foreslopes have to be used, consider installing safety barrier Note also that certain soils that may be present at subgrade may be unstable at 1:1.5 side slopes, and for these soils a gentler slopes may be required. Slope configuration and treatments in areas with identified slope stability problems should be addressed as a final design issue.

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Table 7-4: Slope Ratio Table (Vertical to Horizontal ratio)

Height of Slope (m) Fill side slope V:H 1:4 1:2 1 : 1.5 1:4 1:2 1 : 1.5 1 : 1.25 1:1 1 : 1.5 1:1 1:4 1:2 1 : 1.5 Cut Back Slope V:H 1:3 1:2 1 : 1.5 1:2 1 : 1.5 1:1 3:1 4:1 3:1 3:1 1:3 1:2 1 : 1.5 1 : 4* 1:2 1 : 1.5 1 : 4* 1:2 1 : 1.5 1 : 1.25 1:1 1 : 1.5 1:1 1:4 1:2 1 : 1.5 Recoverable Critical Critical Recoverable Critical Critical Critical Critical Critical Critical Recoverable Critical Critical Side Slope Type of slope

Material type

Earth or Soil

0.00 -1.00 1.00 -3.00 over 3.00 0.00 1.00 1.00 3.00 over 3.00 0.00 2.00 over 2.00 0.00 2.00 over 2.00 0.00 1.00 1.00 3.00 over 3.00

Compacted Lateritic Soil Hard Rock Weathered Rock

Decomposed Rock

*in hilly, mountainous and difficult areas 1:4 could be reduced to 1:2

7.7 DRAINAGE CHANNELS

Good drainage is essential to protect the road from damage. Drainage channels include (a) road side channel running parallel to the road and in cut sections to remove water from the road cross section; (b) toe of slope channel to convey the water from any cut section and from adjacent slopes to the natural watercourse; (c) intercepting channels placed back of the top of cut slopes to intercept surface water; and, (d) chutes to carry collected water down steep cut or fill slopes. Drainage channels perform the vital function of collecting and conveying surface water from the road right-of-way. Therefore, drainage channels should have adequate capacity for the available peak runoff, should provide for unusual storm water with minimum damage to the road, and should be located and shaped to avoid creating a potential conflict with traffic. Channels should be protected from erosion with the lowest cost protective lining that will withstand the flow velocities expected. Channel should be kept clean and free of material that would lower the capacity of the channel. Channel deterioration can reduce channel capacity and overflow may occur. Minimum ditch dimensions is given as follows. Minimum depth of ditches should be 0.6m in mountainous and escarpment terrain, and 1.0m elsewhere, using a U-ditch configuration with bed width of 0.5m. The side slope and back slope of ditches should generally conform to the applicable slope ratio of the road given in Table 7-4. The capacity of ditches should be checked against the available flood discharge and should be ascertained that it can accommodate without being over-flooded or over-topped. In case that the capacity is found to be low, then the size should be revised by either increasing the bed width or making deeper or both. Side drains should be avoided in areas with highly expansive clay soils such as black cotton soils. Where this is not possible, they shall be kept at a minimum distance of 47.7 Ministry of Works, Housing and Communications

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6m from the toe of the embankment. The ditch in such cases should have a trapezoidal, flat-bottom configuration. The minimum recommended embankment height in such flat, marshy and black cotton soil area is 1.5m. In populated areas deep and open road side drain channel are hazardous to vehicles & pedestrians. It is therefore recommended to use covered drain channel or under drain system depending on the size of the town, design class of the road, as well as the construction and maintenance cost of said drainage. Key points to consider in the design of safe side drains are: There should be sufficient discharge points and culverts to ensure that the drain never gets very deep; Open drains are best located outside the clear zone; With open drains, the slope next to the road should as much as possible be flat enough to reduce the risk of errant vehicles overturning; In built-up areas channel drains deeper than 250mm should be covered or underdrain system be used for the safety and convenience of both pedestrians and vehicles; The drain should terminate or discharge in a satisfactory manner without risk of causing erosion or other problems; and, The drain should be capable of being cleaned and maintained easily.

It is not always easy to design drains that can cope with the expected flow and yet are safe, affordable and easy to maintain, so compromises are often required.

7.8 CLEAR ZONE

The clear zone is a safety zone adjacent to the traffic lanes. It is an unobstructed, relatively flat area provided beyond the edge of carriageway for the recovery of errant vehicles. The clear zone must be kept free of rigid objects (such as posts, trees etc with a diametreover 0.10m) and other hazards, such as steep slopes, open drains, etc. Elimination of roadside furniture or its relocation to less vulnerable areas are options in the development of safer roadsides. If the hazard cannot be removed or relocated from the clear zone it should be shielded by safety barrier. If shielding by safety barriers is not possible, for whatever reason, consideration should be given to delineating the feature so that it is readily visible to a motorist. Once a vehicle has left the roadway, an accident may occur. The end result of an encroachment depends upon the physical characteristics of the roadside environment. Flat, traversable, stable slopes will minimize overturning accidents, which are usually severe. For adequate safety, it is desirable to provide an unencumbered roadside recovery area that is as wide as practical on a specific road section. A clear zone of 3 m or more from the edge of the carriageway appropriately graded and having gentle slopes and rounded cross-sectional design, is desirable. At existing pipe culverts, box culverts and bridges, the clearance cannot be less than the roadway width; if this clearance is not met, the structure must be widened. New pipe and box culvert installations, and extensions to same, must be designed with a clearance at least the width of shoulder from the edge of the carriageway. Minimum desirable and maximum horizontal clearance to road signs as well as overhead clearance between overhead structures shall be as per the Ministry Traffic Sign Manual and as reproduced in figure 7-4.

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B A H D D G

N o sh o u ld e r

S h o u ld e r

C D C

F A

E F o o tw a y

K erb

K erb

Minimum (mm) A B C D E F G H 600 1000 300 1000 2100 5000 750

Desirable (mm) 1000 1500 600 1800 2100 5200

Maximum (mm) 2500 2500 2500 2500

5000

Figure 7-4:

Heights and clearances

Horizontal clearance from guard rails, guide posts, and marker posts in rural section shall be a minimum of 0.6 m from the shoulder edge (or carriageway edge if there is no shoulder). However there must be at least 0.6 m between the back of the post and the break of fill slope in order to have sufficient ground support for the post. Maximum allowed vehicle height in Uganda is 4.0 m and minimum vertical clearance of 5.0 m should be allowed for the design. . This minimum vertical clearance could be reduced to 4.5 on Class D and E roads. It is obvious that the need for clear zones increases with speed and curvature. The following clear zone widths (Table 7-5), measured from the edge of the traffic lane, are considered to give an acceptable standard of safety. Traffic volume is also a factor, as, generally, the higher the traffic volume the greater the frequency of run-off-road incidents which supports the use of wider clear zone widths.

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Table 7-5:
Speed limit 70 80 100

Clear zone widths

Standard Desired 5m 6m 9m Minimum 3m 4m 6m

The clear zone widths given in Table 7-5 should be increased at sharp bends on highspeed roads by a correction factor to be obtained from Figure 7-6 depending on the radius of curve. Foreslopes steeper than 1:3 cannot be counted as part of the clear zone because they are too steep. Slopes that can be traversed safely by out-of-control vehicles need to be at least 1:4 or gentler. Slopes between 1:3 and 1:4 are marginal; the normal practice is that half the width of these slopes is counted as part of the clear zone see Figure 7.5.

A v a ila b le c le a r z o n e : 1 .5 + 4 .0 + 0 .5 * 3 .0 + 2 .0 = 9 m 1 :2 1 :4 1 :3

1 .5

1 .0

4 .0

3 .0

2 .0

Figure 7-5:

Example how to calculate clear zones

Clear zone correction factor

Speed limit 100 Speed limit 80

700 m radius and speed limit 100 gives 1.6 and desired clear zone extended from 9 m to 1.6 * 9 = some 14 m Radius (m) 0 300 400 500 600 700 800 900 1000 1100 1200

Figure 7-6:

Clear zone correction factor for bends

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7.9 SIDE ROADS AND CULVERTS

Side roads are often built up on a little embankment so that they enter the main road on the same level. This embankment can be an obstacle to vehicles that run off the road. And the culvert carrying the main road drain under the side road will often have a large, solid headwall. Where there is a culvert under the main road, the culvert headwall is often close to the edge of the carriageway, especially if the road has been widened at some stage. These are hazards. With side roads it is best to try and construct gentle embankment slopes and move the culvert further away from the main road. In the case of the culvert under the main road it should be extended in order to move the ends away from the carriageway edge. It is also important to assess whether culverts really need large solid headwalls. It may be possible to provide a smooth opening instead.

7.10 RIGHT-OF-WAY
Right-of-ways, or road reserves, are provided in order to accommodate the ultimate planned roadway, including all cross sectional elements and to enhance the safety, operation and appearance of the roads. The width of right-of-way depends on class of the road, the cross section elements of the road, topography and other physical controls together with economic considerations. Although it is desirable to acquire sufficient right-of-way to accommodate all elements of the cross section and appropriate border areas, right-of-way widths should be limited to a practical minimum in both rural and developed areas affecting the economy of the inhabitants. Figure 7-7 shows the cross-sectional elements to be considered when determining right of way. A uniform width of right-of-way may be convenient, but there are special cases where additional right-of-way may be desirable. These special cases could be locations where the side slopes extend beyond the normal right-of-way, where greater sight distance is desirable, at intersections and junctions and for environmental considerations. In all cases the right-of-way should always be determined and shown on the final design plans of road projects.

Figure 7-7:

Right of way of road

Road reserve widths applicable for the different road classes are given in Tables 4-3 up to 4-9. In mountainous or escarpment terrain, a cut section may be of such depth that the right-of-way width is exceeded from the top of cut on one side to the other top of cut. Additional areas required for outlets etc., should be provided in a manner that will not endanger the future integrity of the drainage facility and will provide adjoining land owners restricted use of this land after completion of the road. 7.11 Ministry of Works, Housing and Communications

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Reduced widths should be adopted only when these are found necessary for economic, financial or environmental reasons in order to preserve valuable land, resources or existing development or when provision of the desirable width would incur unreasonably high costs because of physical constraints. In such cases, it is recommended that the right-of-way should extend a minimum of a nominal 3 metres from the edges of the road works. However, where this occurs, it is advisable to restrict building activity along the road to prevent overcrowding, to preserve space for future improvements, and to allow provision for sight distances at curves.

7.11 FOUR-LANE AND DIVIDED ROADS

In the case that the traffic volumes could not be accommodated by a two lane rural road then there will be a need to increase the roadway to a four-lane facility when a certain volume is reached. It is also the case that some cities and villages have included a four-lane roadway as a feature in their master plans. Four lane and divided roads are required when the design traffic volume is sufficient to justify their use. They are also frequently used in urban/peri-urban areas. As discussed earlier a minimum median width of 4.8 or about 5.0 metres is required to allow the provision of right-turning lanes outside of the adjacent carriageway, and to avoid having a turning passenger vehicle from distributing the vehicles on the through lanes.

7.12 SINGLE LANE ROADS

For low traffic volume roads (<50ADT), single lane operation is adequate as there will be only a small probability of vehicles meeting, and the few passing maneuvers can be undertaken at very reduced speeds using either the shoulder or passing bays. In such cases adequate sight distances should be provided for safe stopping, these maneuvers can be performed without hazard, and the overall loss in efficiency brought about by the reduced speeds will be small, as only a few such maneuvers will be involved. The lowest design classes or road will not allow passing and overtaking to occur on the carriageway and passing must be performed using shoulders. In such cases the width of roadway including shoulder should be enough to allow two design vehicles to pass, i.e. a minimum of 6.0 metres width, and vehicles would be expected to stop or slow to a very low speed. In the case where shoulders are not provided passing and overtaking maneuvers are to be undertaken on passing bays. The increased width of road at passing bays should be enough to allow two design vehicles pass safely. In such cases it is normal that passing bays should be located every 300 to 500 metres depending on the terrain and geometric conditions. However, adjacent passing bays must be inter-visible and appropriately placed, inter-visible passing bays are essential to ensure the free flow of traffic. Account should be taken of sight distances, the likelihood of vehicles meeting between passing bays and the potential difficulty of reversing. In general, passing bays should be constructed as the most economic locations as determined by terrain and ground conditions, such as transitions from cuttings to embankment, rather than at precise intervals. The length of individual passing bays will vary with local conditions and the type of design vehicle but, generally, a length of 20 metres including tapers will cater for most commercial vehicles.

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Section 7

7.13 MEDIAN
A median is highly desirable on road of four or more lanes. A median is defined as the portion of a divided road separating the carriageway for traffic opposing directions. The median width is expressed as the dimension between the through-lane edges and includes the right shoulders, if any. It includes the inner shoulders and the central island. The principal functions of a median are to separate opposing traffic, provide a recovery area for out-of control vehicles, provide a stopping area in case of emergencies, allow space for speed changes and storage of right-turning and U-turning vehicles, minimize headlight glare, provide width for future lanes, and to provide a refuge for pedestrian crossing the road in case of urban and populated areas. For maximum efficiency, a median should be highly visible both night and day and contrast with the through traffic lanes. Medians may be depressed, raised, or flush with the carriageway. Medians should be as wide as feasible but dimension in balance with other components of the cross section. In determining median width, consideration should be given to the possible need for median barrier. Where possible, median width should be such that a median barrier is not warranted. In general, the median should be as wide as practical. Where the median is less than 9.0 m wide consider installing a median barrier. However, economic factor and available of land (right-of-way), and also terrain often limit the width of median. The minimum width of median is as narrow as 1.2 to 1.8 m. Where provision for right turn lane is required the minimum width of median will be 4.8 5.0 m. In some cases for future upgrading of the road a central reserve of minimum 12.0 m could be introduced to serve as median. The 12m central reserve could accommodate in the future two lanes each having 3.5m in addition to a 5.0 m median. Figure 7-8 gives recommendations on median design and width on high-speed rural dual carriageway roads. Medians on urban dual carriageways should normally be designed to function as a refuge for pedestrians. The median should have a minimum width of 2.0m, but this can be reduced to an absolute minimum of 1.2m where space is very restricted. A 2.0 m width will also give sufficient space for most signs, signals and lighting columns. Median barriers should not normally be necessary on urban dual carriageways with speed limits of less than 80km/h.
If e m b a n k m e n t
1 :4 1 :4

In n e r h a rd s h o u ld e r

M e d ia n w id th s p e e d lim it d e s ira b le m in 100 9 6 80 6 4

Figure 7-8:

Median designs at speed limit 80 and 100 km/h

7.13 Ministry of Works, Housing and Communications

Geometric Design Manual

Section 7

7.14 CROSS-SECTION OVER BRIDGES AND CULVERTS

The safety and operational capacity of bridges and culverts will be affected if the road cross-section is not maintained across these structures. The key points to consider with respect to cross-sections over bridges and culverts are: Any significant narrowing of the traffic lane or shoulder is dangerous, especially on high-speed roads; When roads are being upgraded and widened, the bridges and culverts will normally need to be widened as well; If the shoulder is not continued across the structure, vulnerable road users will move out into the traffic lane in front of fast-moving vehicles and there will be a risk of collisions; Footways are conventionally provided on structures with parapets, but the accident risks need to be assessed carefully, especially where footways can only be provided by omitting the shoulders; take account of the relative volumes of pedestrians and cyclists, the speed and volume of motorised traffic and the length of the span; Where footways are provided they should be a minimum of 1m wide, and they will usually be quite high (150-200mm) in order to better protect the bridge parapet, in which case the ends must be stepped or ramped down and flared away from the edge of the traffic lane; It is best to segregate the vulnerable road users from vehicles by means of a safety barrier see Figure below and this is the preferred solution for bridges on Class A and Class B roads; Where, exceptionally, a single lane bridge is planned the traffic lane should be a maximum of 3.7 m wide between kerbs in order to avoid confusion over whether the bridge is for one-way or two-way traffic.

See Section 12 for advice on the design of bridge parapets

P e d e s t r ia n p a ra p e t

R e in f o r c e d c o n c r e t e s a fe ty b a r r ie r

1m

F o o tw a y 2 m m in im u m C a r r ia g e w a y

Figure 7-9:

Segregated footway on bridges

(Note: if cyclists are likely to use the footway increase the parapet height to 1.4 m)

7.15 FOOTWAYS

AND CYCLEWAYS

The conventional practice is to assume that pedestrians and cyclists can use the shoulders, but it is much safer for them to be on a separate footway, or combined footway/cycleway. At high flows there can be conflicts between cyclists and pedestrians, but these are not as dangerous as conflicts with motor vehicles. Combined footways/cycleways should be 3.0m wide (2.0m absolute minimum). It is

7.14 Ministry of Works, Housing and Communications

Geometric Design Manual

Section 7

important for footway and cycleway surfaces to be at least as smooth as the adjacent traffic lanes and shoulders preferably smoother. See also Section 11.

7.15.1 Footways and cycleways in rural areas

The footway/cycleways should be separated from the carriageway by a grass strip or similar, at least 2.0m wide. Gradients should be gentle, preferably less than 4%. On embankments the footway/cycleway can be benched into the fore slope. The planting of shade trees can help encourage people to use the facility.

c a rr ia g e w a y

F o o tw a y / c y c lew a y

s h o u ld e r

tr a ffic la n e

tra ff ic la n e

s h o u ld e r F o o tw a y / c y c le w a y

d iv id in g g ra s s s trip o r s im ila r

Figure 7-10:
7.15.2

Separate foot and cycleway on rural roads

Footways and cycleways in built-up areas

Raised, kerbed footways should be provided in the larger built-up areas. Cycleways, where necessary, should be constructed behind the footway.

F o o tw a y > = 2 .0 m

C a rria g e w a y 6 - 6 .5 m

F o o tw a y > = 2 .0 m

Figure 7-11:

Raised, kerbed footway in urban areas

A simpler and cheaper alternative is to have the footway at the same level as the traffic lane, but separated by a barrier kerb or low wall see Figure 7-12. This means it can function as a combined footway/cycleway. Gaps are left in the separator to allow drainage and access to roadside premises. The separators should be painted white to make them more visible at night, and care should be taken to avoid starting the separator where speeds are high or visibility is poor. If necessary, fit reflectors to the end of the separator.

7.15 Ministry of Works, Housing and Communications

Geometric Design Manual

Section 7

F o o tw a y /c y c le w a y > = 2 .0 m

C a rria g e w a y 6 - 6 .5 m

F o o tw a y /c y c le w a y > = 2 .0 m

s e p a ra tin g k e r b o r s im ila r

s e p a ra tin g k e r b o r s im ila r

Figure 7-12:

Footway/cycleway on physically separated shoulders

7.16 SERVICE ROADS

In the larger trading centres and towns it is recommended that service roads be provided. A typical service road design is illustrated below. The local access traffic is kept separate from the through traffic, and the service road provides space for parking, unloading and loading, bus stops, and informal trading.

F o o tw a y > = 2 .0 m

M a in ro a d

F o o tw a y > = 2 .0 m

S e rv ic e ro a d ty p ic a lly 6 .0 m

Shop

D ra in

D ra in
Figure 7-13: Service roads

7.17 TYPICAL CROSS SECTIONS

Typical cross sections for road classes Paved Ia, Ib, II, III, and Gravel A, B, & C for both Town and Rural (Normal) sections are given in the following figures (7-14 7-18). It is very dangerous for pedestrians and cyclists to use the shoulder of high-speed roads. Therefore the separate foot and cycleways are required to be introduced. The introduction of footways and cycleways in rural and urban areas, when they are required to be introduced, shall be as per Section 7.15 of this manual. It is preferable that high class roads the primary function of which, are to provide mobility (such road design classes of Paved Ia & Ib), do not pass through towns and populated areas. In roads passing through towns, in addition to the basic or through lanes, an outer lane on both sides of the road are to be introduced in order to avoid disruption of nonstopping or passing traffic by stopping vehicles in the towns. Provision of parking lane on higher class of roads in towns may have adverse impact on safety of pedestrians. Therefore segregation of through traffic from parking and footways is preferable for safety

7.16 Ministry of Works, Housing and Communications

Cut
Road Reserve Width (RRw) Road Way Width (RWw) Shoulder (Sw) 0.30 Lane Width (Lw) 0.30 Lane Width (Lw) Carriage Way Width (Cw) Shoulder (Sw)

Fill

CL
Ssl Csl Ssl Csl

Varible Slope ratio See section 7

Geometric Design Manual

Varible Slope ratio See section 7

1.00

Varible Slope ratio See section 7

0.50

Natural Ground Level

Ministry of Works, Housing and Communications Dimension(m) RRw 40.0 30.0 30.0 30.0 25.0 15.0 6.4 8.6 10.0 6.0 5.6 4.0 8.6 5.6 10.0 6.0 2 2 2 2 1 11.0 7.0 2 3.0 2.8 3.0 2.8 4.0 RWw Cw No of Lane Lw 3.5 2.0 1.5 2.0 1.5 1.2 Sw 2.0 2.5 2.5 4.0 4.0 4.0 Csl 2.5 Slope (%) Ssl 4.0 4.0 4.0 4.0 4.0 4.0

Design Class of Road

Paved Ib

Paved II

Paved III

Gravel A

Gravel B

Section 7

Gravel C

7.17

Figure 7-14: Typical normal cross section and values of various cross section elements for Road Design Classes Bitumen Ib, II, III, Gravel A, B, and C

Geometric Design Manual

Ministry of Works, Housing and Communications

(OLw)

Fo t waywidth (FWw)

Outer Lane width

F
sl sl

sl

(OLw)

OL F

sl

B/TL OL

Basic / Trough Lane width

sl

(B/TLw)

RRooaaddRWeasyerWveidWthi(dRthW(wR) W) Car iage WayWidth (Cw) Fo t waywidth (FWw)

Basic / Through Lane width

(B/TLw)

B/TL

sl

Outer Lane width

Slope (%)Fsl4.04.04.0 OLsl 4.04.04.0 B/TLsl 4.04.04.0Dimension (m)FWw2.52.02.0OLw3.02.52.5 B/TLw3.02.84.0No of lane221 Cw 12.010.69.0 RWw 17.014.613.0RRw302520Desig n Class of RoadGravel AGravel BGravel C

Section 7

7.18

Figure 7-15:

Typical town cross section and values of various cross section elements for Design Class Gravel A, B, and C roads

Geometric Design Manual

Section 7

SlopeFsl (%)3.03.03.0 OLsl 3.03.03.0 B/TL(%) 2.52.52.5Dimension(m)FWw 2.52.52.5OLw3.53.02.5B/TLw 3.53.02.8 C 14.012.010.6 RWw 19.017.015.6RRw403030Desi gn Class of RoadPaved IbPaved IIPaved III

Fo t way width (FWw)

sl

7.19

Ministry of Works, Housing and Communications

OuterLane width (OLw)

OL

sl

Figure 7-16:

Typical town cross section and values of various cross section elements for Design Classes Paved Ib, II and III roads

Geometric Design Manual

2.5 3 %
Fo t way width (FWw)

Travel anewidth 3.5

% 3 %
F
sl

2x3.65
OuterLane width

RoadwaywidMtehdianwdth 1.2-5.0 (min) Outerlanewidth 2.5 Fo twaywidth

C31.60
OL
(OLw)
sl

Travel anewidth

2x3.65

2.5

Besic/ThroughLane Width

B/TL

sl

(TLw)

RRooaaddWRaeyseWrvidetWih(RdWthW(R) W) Car iageWay Width(C)

Besice/ThroughLane Width

(TLw)

Ministry of Works, Housing and Communications

Figure 7-17:

Typical Normal Cross Section for Design Class Paved Ia Road

RoadReserve (Rightofway)

Shoulder 2.5

e ratio Se section 7 Natural Ground Level (NGL)

3% Varible Slope ratio Se section 7

0.30

Through Lane Width

2 .5 % 3%

2x3.65 7.30

RoadWay Median 1.2 - 5.0

Through Lane Width

2x3.65 7.30

2 .5 %

0.30

Shoulder 2.5

Section 7

7.20

Figure 7-18:

Typical Town Cross Section for Design Class Paved Ia Road