Sultanate of Oman.

Highway Design Standards 2010

5
Junction Selection
and Capacity Considerations

5.1 General Selection Considerations 5-1

5.1.1 Introduction 5-1
5.1.2 Design Objectives 5-1
5.1.3 Junction Types 5-1
5.1.4 Junction Spacing 5-2

5.2 design principles 5-2

5.2.1 Introduction 5-2
5.2.2 General Considerations 5-2
5.2.3 Geometric and Operational Requirements 5-2
5.2.4 Safety 5-3
5.2.5 Pedestrian Provision 5-3

5.3 Junction Evaluation 5-3

5.3.1 General 5-3
5.3.2 Operational Issues - Capacity 5-3
5.3.3 Operational Issues - Delay 5-4
5.3.4 Queuing Delays 5-4
5.3.5 Geometric Delay 5-4
5.3.6 Resource Costs 5-4
5.3.7 Junction Detailed Design 5-4

5.4 References 5-4

List of figures
Figure 5.1.3.1: Type of Junction Suitable for Varying Traffic
Flows on Major and Minor Roads 5-1

List of Tables
Table 5.3.1.1: Junction Levels of Service According to
Average Delay 5-3
 Sultanate of Oman. Highway Design Standards 2010

Junction Selection and Capacity Considerations

5
5.1 GENERAL SELECTION CONSIDERATIONS would be an acceptable delay at a particular junction location. This may be related to a wider network strategy
where delay might be increased in some cases where movements are discouraged, and decreased in other cases
5.1.1 Introduction where movements carry public transport.

5.1.1.1 Junctions are one of the most crucial components in a highway transport system, being constraints 5.1.2.5 Fuel consumption is an important consideration given that significant reductions in fuel use can be
to link flows where delay, accidents and emissions tend to be concentrated. Determining the optimum junction achieved by reducing stop start conditions and overall delays with consequent economic, noise and air pollution
design for a specific set of conditions can be complex and in most cases require the use of computer software reduction benefits.
to analyze alternative designs with various patterns of traffic demand to determine flows, capacities and delay.
5.1.2.6 For most large junctions where capital expenditure is high, an economic appraisal should show that the
Nevertheless, engineering judgment combined with knowledge and experience of the designer, still remains the
capital costs of the scheme are more than covered by the economic benefits derived.
most important factor in arriving at the most suitable solution.

5.1.1.2 The basic role of a junction is to facilitate the safe and efficient transfer of traffic streams from one side 5.1.3 Junction Types
of a junction to the other. Transfers can be between roads of the same or different levels in the hierarchy (See
5.1.3.1 Various junction types provides a natural hierarchy that cater for increasing levels of traffic flow. These
Section 1), but usually by not more than between adjacent levels. Access roads for instance should not connect
are:
with primary roads.
n Priority controlled junctions (See Section 6)
5.1.1.3 Junction conflicts can be categorized for road traffic as being diverging, merging or crossing, but n Priority controlled junctions with partial or full channelization (See Section 6)
further categories can exist between vehicle classes, for instance cyclists and buses, and between pedestrians
n Roundabouts (See Section 7)
and vehicles. All of which must be considered and balanced in the junction design process.
n Roundabouts with part-time or full- time control by traffic signals (See Section 7)
5.1.1.4 The primary features to be considered in the design of road junctions are that: n Conventional traffic signal control (See Section 8)

n The capacity of the road network largely depend on the capacity of the junctions, particularly in n Grade separated junctions (See Section 9)
urban areas,
5.1.3.2 Figure 5.1.3.1 (Ref 1) illustrates broadly the typical level of flows of major and minor road traffic that can
n The majority of serious injury road accidents occur at junctions, be handled by particular types of junctions. This is a generalized figure, which is a reasonable first guide as to the
n Conflict between pedestrians and vehicles are greatest at junctions. type of junction which should be planned. Specific site circumstances, however, may result in boundary shifts
between junction types, and therefore recognition of this possibility must be made. Predominant influences are
5.1.2 Design Objectives the pattern of movement through the junction, particularly the relative numbers of left turning traffic and any
special geometric characteristics.
5.1.2.1 In brief terms, the design objectives for junctions should be to minimize the following:

n Traffic accidents
n Traffic congestion
n Traffic delay
n Fuel consumption
n Air pollution and noise

5.1.2.2 Accident risk should be minimized particularly for vulnerable users, whilst design should attempt to
reduce the severity of accidents as far as practical. Safe and convenient passage for pedestrians and cyclists is
necessary, especially so in urban conditions for people with visual or mobility impairment.

5.1.2.3 Capacity should be adequate for the particular situation, designed with a view to achieving compatibility
with the overall delays and levels of service on approach roads, particularly the mainline approaches. Conflicts
in the junction area between junction users and traffic related to existing or planned developments in the vicinity
should be minimized.

5.1.2.4 For unsaturated junctions, geometric delay is likely to be the main design delay factor under Figure 5.1.3.1 Type
 of Junction Suitable for Varying Traffic Flows on Major and
consideration, whilst queuing and general congestion delay will predominate at heavily trafficked junctions. Even Minor Roads

small delays at junctions can accumulate over relatively short periods of time to significant levels in economic
terms, particularly for heavily used urban situations. Care must therefore be exercised when assessing what

5-1
 Ministry of Transport & Communications . DGRLT

Junction Selection and Capacity Considerations

5.1.4 Junction Spacing 5.2.2.6 Should the junction or junctions be part of an area wide traffic control system, or is likely to form part of
a future system, (See Section 8) then due allowance for this must be made in the preliminary stages of the design
5.1.4.1 Generally, spacing is determined by the level of the road in the hierarchy (see Section 1) and proportion process.
of through traffic (non local) which it is intended to carry, and the nature and frequency of intersecting roads.

5.1.4.2 Should the number and importance of exiting cross routes necessitate multiple, closely spaced junctions, 5.2.3 Geometric and Operational Requirements
consideration should be given to combining roads before they meet the main road providing benefits in relation
5.2.3.1 Layouts for junctions should be designed to provide the heaviest traffic flows with the easiest and
to road safety and main road capacity.
shortest paths, but in such a way that does not encourage high speeds. The resulting smooth and relatively slow
5.1.4.3 Junctions should be spaced at regular intervals, where possible, so that driver expectation is reinforced passage through a junction will enhance safety aspects particularly for vulnerable users. The introduction of
and they are associated with a particular class of road. Spacing should be at the very minimum, just greater than sharp turns, or complex paths which include several changes of direction should be avoided wherever possible.
the stopping sight distance of the 85th percentile speed on the major road. Drivers should know what to expect in respect of traffic/traffic and pedestrian/traffic conflicts when entering a
junction having recognized its layout and taken adequate note of approach signing.

5.2 DESIGN PRINCIPLES 5.2.3.2 Junction layouts should be designed with due allowance for vehicle swept paths and will be determined
largely by the typical (most common for that location) type of commercial vehicle that might reasonably use the
5.2.1 Introduction facility. This is not necessarily the longest vehicle. In high end junction design this is conventionally taken as a
semitrailer or draw bar trailer, and a fixed axle bus for lower order routes where large commercial vehicles would
5.2.1.1 The principles of good junction design can be listed under the following headings: not typically penetrate but bus service and utility vehicles would. Junction designs should allow for occasional
n General Considerations encroachment in lower order roads where considered appropriate.
n Geometric and operational requirements of vehicular traffic
5.2.3.3 Primary design principles are to reduce or separate conflicts as much as reasonably possible. Choice
n Safety
of layout will determine the numbers and degree of conflicts. Roundabouts result in the least number of conflicts
n Provision for pedestrians eliminating crossing conflicts entirely. Conflicts can be managed by providing channelization using raised curbs
or by utilizing painted Ghost Islands. Extensive use of small islands at junctions can however become ineffective
5.2.2 General Considerations and confusing for drivers and should be avoided.

5.2.2.1 The layout and control aspects of junctions should be considered together to determine the best design 5.2.3.4 Channelization at junctions which can separate merging, crossing and diverging conflicts should be
solution to a particular problem, with the fundamental goal of providing road users with consistent standards designed at such a scale to allow drivers in adequate time to understand the nature of the streams of traffic and
which would not be confusing. On lengths of road under design, use of a mixture of junction types and control clearly see which streams they will or will not conflict. Only under these conditions will the gaps in traffic be
treatments is likely to create uncertainty for drivers and therefore increase the risk of accidents. Similarly for utilized effectively and result in full operational efficiency.
pedestrians, consistent treatment helps the user to understand and therefore use the system appropriately.
5.2.3.5 It is critical that ALL road users have adequate visibility at junctions including pedestrians as well as
5.2.2.2 Major/Minor priority layouts are the most common form of control allowing main road traffic to progress drivers. This is to ensure that conflicting traffic movements can be seen in sufficient time so that appropriate
without delay. This however is only suitable for lightly trafficked roads and also with the proviso that main road action can be taken to avoid accidents. This principle applies to all junctions irrespective of size or type.
speeds are not excessive and overtaking not allowed in the area.
5.2.3.6 Junction location should be restricted wherever possible to locations where mainline or side road
5.2.2.3 Higher traffic levels warrant higher junction provision with more complex layouts and more sophisticated profiles are not in significant crest curve. To increase driver’s forward visibility of junction layouts, it is always
traffic controls. In such cases, given the uncertainty of traffic predictions, designers should always consider if preferable to locate junctions on level approaches or within sag curves. Uphill approaches to a junction make it
stage construction would be appropriate, with built-in provision for upgrading at a later date if traffic levels difficult for drivers to understand the layout and should be avoided where possible.
warrant.
5.2.3.7 Speed differentials between mainline traffic and turning (merging and diverging) traffic are undesirable
5.2.2.4 Design of junctions, particularly grade separated junctions, needs careful consideration of the direction and should be reduced by providing merge or diverge lanes with tapered entrance/exits. Provision of a storage
of priority. Normally associated with the heaviest traffic flow demand, this situation is sometimes controlled by lane for left turning traffic off a mainline is another safety and operational measure that can also improve junction
the overall network strategy. effectiveness, eliminating conflict with mainline “through” traffic coming from behind. In cases where right
turning movements predominate at junction approaches affecting capacity for through and left turning traffic,
5.2.2.5 Where a major road is to provide a high level of service, a corresponding level of service for junctions
exclusive segregated right turn lanes should be considered.
should be consistently offered for mainline traffic along that route. It may be that some predicted junction flows
do not in numerical terms warrant this treatment. However, to maintain “driver expectation” for a particular class 5.2.3.8 The design of road signing and road marking is an integral part of the design process for junctions and
of road with a particular level of junction provision, consistency of design is essential. should receive serious attention to ensure that they fulfill their role effectively. Layouts should be checked to see
that they can be adequately signed without ambiguity, that clear advance signing is provided and that signing
does not affect junction visibility in any way. (See Section 19 of these Standards).
5-2
 Sultanate of Oman. Highway Design Standards 2010

Junction Selection and Capacity Considerations

5
5.2.4 Safety 5.3.1.2 In cases where junction design is not constrained by other factors, relative assessments of safety,
performance, operational efficiency and resource costs can be used to assist in the choice of the preferred
5.2.4.1 Safety considerations should remain as the core element for all junction design, from initial concept to design option. Evaluation should cover the effects at other junctions and links caused by the proposals within a
final design and layouts should, when instructed by the Client Authority be subjected to independent audit during highway network across a pre-identified “Area of Influence.” Conventional economic assessment should include
the design process. Critical factors in the realization of a reduction in occurrence and severity of accidents at a comparison of network wide parameters for vehicle-km and delay, applying appropriate values for time and
junctions are: vehicle operating costs to ultimately determine economic parameters such as FYRR, EIRR and NPV for scheme
n A reduction in high vehicle speeds through the junction alternatives.
n The presence of unobstructed visibility on all approaches to the junction
5.3.1.3 Traffic forecasting in absolute terms, particularly long range forecasting, is not a precise science, and
n Appropriate geometric standards for the typical design vehicle, and
to achieve a good level of accuracy is not always possible, even when using sophisticated modeling tools.
n Integration of the traffic information and control systems within the junction
Junction evaluation, however, provided that consistent traffic figures are used for testing, can reveal the true
5.2.4.2 Whilst there remains flexibility in the planning process to design junctions appropriate to specific relative ranking of the alternatives looking at the life of the scheme and the effect of proposals over the area of
conditions, the following general recommendations hold: influence.

1) It is not recommended that traffic signals be used on high speed roads (posted speeds greater than 80km/h) 5.3.1.4 In order to further ensure that there is a consistent approach to economic analysis of junctions in Oman,
based on safety grounds. the adoption of suitable values for Time and VOCs shall require presentation to and verification by the Overseeing
2) It is not recommended that at-grade roundabouts be used on high speed roads (posted speeds greater than Authority before use.
100km/h) based on safety grounds. An exception to this case would be the terminal points for the high speed
road where roundabouts are a safe provision and particularly useful to introduce a change of road hierarchy 5.3.1.5 General guides to the operational efficiency of junctions, both signalized and non-signalized are shown
or route classification, in Table 5.3.1.1. This uses a similar “Level of Service” type approach as used in Section 4 of these standards –
“Road Link Geometry”. Average delay per vehicle is the usual measure of junction operational success. This can

5.2.5 Pedestrian Provision be applied over specific time periods either to a particular movement or selective traffic movements or for all
traffic using the junction. As noted earlier, care must be exercised in evaluating junction improvements beyond
5.2.5.1 Pedestrian safety is considered in Section 17, however, a brief word about basic principles at junctions the immediate boundary of those specific sites.
are given. Pedestrian safety should receive careful consideration from the earliest stage of design, especially
for urban and semi-urban areas. It could be possible for designs to provide separate routes for pedestrians Ave. Stopped Delay
Ave. Stopped Delay
Level of Service (LoS) for Signalized Junctions
which lead them away from the busiest parts of the junction to places where road widths are reduced and traffic for Un-signalized Junctions (sec/veh)
(sec/veh)
movements are more predictable. At the junction, one of several provisions could be considered to improve A <=10.0 <=10.0
pedestrian safety via: B >10.0 and <=20.0 >10.0 and <=15.0
n Providing a raised central island refuge at unmarked crossing places C >20.0 and <=35.0 >15.0 and <=25.0

n A pedestrian crossing without signal control with/without a central island refuge D >35.0 and <= 55.0 >25.0 and <= 35.0
E >55.0 and <= 80.0 >35.0 and <= 50.0
n A signal controlled crossing; or
F > 80.0 > 50.0
n A grade separated crossing (pedestrian underpass or bridge)
Table 5.3.1.1 Junction Levels of Service According to Average Delay
5.2.5.2 Choice of the above facilities is dependent on predicted traffic volumes, and the pattern of movement
5.3.1.6 Table 5.3.1.1 indicates the best LoS “A” being with virtually no traffic based delay down to LoS “F”, which
of both vehicles and pedestrians, and should be designed in accordance with Section 18 and other standards
is conventionally considered to be at unacceptable levels and associated, for example, at signalized junctions,
provided in this document and their associated references.
with very long cycle times. (Ref 2)

5.3 JUNCTION EVALUATION 5.3.2 Operational Issues - Capacity

5.3.1 General 5.3.2.1 Capacity analysis is primarily concerned with the ability of the junction to cope with the various
traffic movements that will be made on it. For priority and roundabout junctions (space sharing), this depends
5.3.1.1 Junction design and evaluation have in-built levels of uncertainty given the difficulties of forecasting essentially on the arrival patterns of the major movements, the junction layout, type of control and gap acceptance
traffic use, especially urban traffic flows and patterns. Traffic flows can change significantly in composition, volume characteristics.
and direction on an hourly and daily basis. The respective operational efficiency of alternative junction designs
will also affect the attractiveness of a junction thus implying an iterative process rather than the application of 5.3.2.2 For isolated signalized junctions (time sharing), the situation is different. There is no dependence on
a simple fixed trip matrix. Care must be exercised, therefore, when choosing the most representative design arrival patterns; there is no fixed priority and no gap acceptance issues for through traffic.
condition, and a variety of design flows should be tested which may lead to a range of proposals that offer
varying levels of service and other advantages or disadvantages.
5-3
 Ministry of Transport & Communications . DGRLT

Junction Selection and Capacity Considerations

5.3.2.3 For all types of junctions, the traffic composition is an important consideration when evaluating 5.3.5 Geometric Delay
capacities, since the distribution of vehicle sizes and associated operational characteristics in a traffic queue or
stream affects capacity. For intersections, capacity is expressed as pcus (passenger car unit) per hour for any 5.3.5.1 The geometric delay is a function of the junction geometry and on the speed of a vehicle as it decelerates

particular movement. A single pcu is the base unit of flow to which all other vehicles types are converted. For a on the junction approach, negotiates the junction and accelerates away on leaving. The total geometric delay for

particular junction type, if a heavy vehicle takes say twice the time of a single passenger car unit to navigate a any flow pattern can be obtained by multiplying the delay per vehicle in each stream by the corresponding flow

particular junction type, the equivalent pcu value for that vehicle would be 2pcus. and summing for all the traffic streams. The resulting aggregated traffic delays, in both the with and without
project cases, are compared in terms of their difference in vehicle operating cost and driver/passenger time, to
5.3.2.4 For priority junctions and roundabouts, it is conventional under normal geometric conditions to assume obtain the net benefits for the with-project case.
that heavy vehicles and large buses are equivalent to two pcus. Similarly, for traffic signalized junctions, the
effects of various vehicle types under saturated conditions can be taken as: 5.3.6 Resource Costs
n Car or light goods vehicle = 1.0 pcu
5.3.6.1 The generalized cost of traffic operation at a junction will consist of:
n Medium goods vehicle = 1.5 pcu
n Time (delay) costs
n Heavy goods vehicle = 2.3 pcu
n Vehicle operating costs, and
n Bus/coach = 2.0 pcu
n Accident Costs
n Motorcycle = 0.4 pcu
n Bicycle = 0.2 pcu 5.3.6.2 For optimization of the junction design, the objective should be to minimize these costs, discounted over
the planned life of the junction, in comparison with the capital cost of the scheme and discounted maintenance
5.3.2.5 In cases of particularly high cross-junction gradients or other extreme geometric constraint, the designer
costs over the life of the junction.
should nominally adjust the standard values quoted here appropriately.
5.3.6.3 Vehicle operating cost (VOC) consist of two elements, fixed and variable costs, with fixed cost comprising
5.3.3 Operational Issues - Delay of items such as license, insurance and taxes, and variable costs, comprising of fuel, tires, oil, maintenance etc
which relate to use. Values of time relate to the trip purpose and type of vehicle, normally linked to the average
5.3.3.1 There are two types of delay that occur at junctions depending on their level of saturation, congestion
wage of passenger car drivers and heavy vehicle drivers.
delay and geometric delay. Congestion delay, arising from queuing, is caused entirely by other traffic and is
dependent on the levels of flow and available capacity on any particular approach. The other type, geometric
5.3.7 Junction Detailed Design
delay, is fixed, and is dependent on the geometry of the junction and control strategies that apply.
5.3.7.1 There are many proprietary programs available for analysis of junctions, some of which attempt to
5.3.4 Queuing Delays include safety requirements as well as capacity estimates. These parameters may sometimes be in conflict, with
lower capacity junctions providing higher safety parameters and vice versa for higher capacity junction designs.
5.3.4.1 For each traffic stream approaching a junction, the queuing delay is dependent on:
In such cases, these conflicts should be reconciled for the situation that prevails at each junction, meaning taking
n The capacity of the junction to pass that traffic under prevailing conditions (i.e. the maximum service a considered and professionally balanced view of all the contributory features to arrive at a preferred solution.
rate of the junction on that approach under prevailing conditions), and
5.3.7.2 Current programs available for the analysis of junctions are noted in respective Junction Design Sections
n The arrival rate of vehicles on that approach.
6, 7, 8 and 9 in which basic geometric design standards are also presented. Typically these programs input design
5.3.4.2 The arrival rate is the average rate (vehs/h) at which the traffic upstream approaches the junction and geometry and traffic movements to calculate performance overall and on individual junction legs, predicting
is measured upstream of any queue. The prevailing capacity is the average rate at which traffic can enter the queue lengths and delays over a series of time slices. They can also be used for checking swept paths of various
junction on the arm under consideration at the design condition. vehicle types on junction designs. These programs should be used, where appropriate, to conduct a series of
tests to ensure the robustness of junction design to accommodate traffic demand.
5.3.4.3 The arrival rate and the prevailing capacity can change significantly, particularly over peak periods
where sudden short peaks within the peak period can occur. Where practicable, the junction design should
5.4 References
provide sufficient capacity to keep the ratio of arrival rate to prevailing capacity to less than 0.9 for each traffic
stream. 1. Transport in the Urban Environment 1997 UK - The Institution of Highways and Transportation
2. Highway Capacity Manual (2000), Transportation Research Board, National Research Council, Washington,
5.3.4.4 Where such capacity buffers cannot be maintained, and ratios are above 1, there will be continual queue
D.C., 2000.
growth until such time that arrival rates drop to levels which can be cleared by that particular junction leg. The
junction design will affect the total amount of queuing delay and the distribution of delay on the various traffic
streams. The design can thus be used to achieve given objectives for the movement and distribution of traffic.

5-4