18-Jan-18

Objective
 To give an appreciation of the most important elements of
Junction Design – the design of junctions of different types
an Introduction
Detailed design and capacity analysis will not be discussed here

General Principles Turning Radii

 Drivers should make one decision at a time and should be given a  Basically, accommodate 15.5m lorries
time to act
 Safely follow natural paths  Widening should be provided for outside overhand and rear
 Separate conflicting movements either by time and/or space wheel “cut-in” of trailers
 Provide easier paths to heavier traffic volume; minor traffic could
be slowed/stopped  Allowance should be made for passing stationary vehicles
 Layout and operation of the junction should be well understood by
the drivers
 Provide adequate waiting areas, avoiding block back
 Provide clear message to driver of the type of junction ahead
 Accommodate 15.5m lorries
 Avoid many traffic islands
 …

Operation Principles

Priority Junctions
aka Two-Way Stop Controlled Junctions

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Priority Junctions - Types Design Principles

 Stop where sight distances are poor
3 Leg 4 Leg
 Junctions should not result in excessive delay
 Plan for future traffic growth without additional land

Direct Staggered Major road

Minor Road
Major Road
Give-way or Stop
Minor Road negotiates junctions when suitable gaps become available in
the major road traffic.
Minor road

Capacity – Main Determinants Visibility

 For minor road:  Important as traffic on side road need to “grab gaps”
 Flow on major road Major road
 Speed on major road
 If a function of major road traffic speed
 Visibility  May be restricted by buildings
 For major road  Locations of junctions should be where visibility is good
 Proportion of vehicles turning right Give-way or Stop
 Heavy left turns
 e.g. Not on a crest of a hill
 provide safe reservoir Minor road  Islands may be used to separate cutting, merging, and
 % of heavy vehicles diverging movements
 Driver Information
 Clarity of layout
 Definitions of paths
 Road markings
 Signing

Roundabouts - Operation Principles

 operate as a one–way circulatory system around a central island,
where entry is controlled by ‘Give Way’ markings and priority is
Roundabouts given to circulating traffic
 efficiency of this type of junction depends on the ability of drivers
to respond to safe opportunities to join the stream of circulating
vehicles already using the junction.

 Design Regimes
 Urban Areas: high peak flows, marked tidal variations, physical restrictions
 Rural Areas: high approach speeds, low tidal variations
 Few physical constraints

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Design Requires Types

 Compromise
 Secure safe interchange  Normal
 Minimize delay  Mini
 Need to match geometric layout to volume and speeds within  Double
local constraints
 Trade Off
 Operational frequency
 Minimizing delay
 Concern for safety and local constraints

Normal Roundabout Mini-roundabout

 One way circulation  Slightly raised circular marking of 125mm maximum dome
 Kerbed central island greater than about 4m diameter height
 Usually flared approaches  With or without flares
 3 or 4 entries (junctions with 5 entries do not usually  Effective in improving existing junctions with capacity/safety
perform well) problems
 Is most efficient balanced flows  Needs extensive signs and warnings
 Large roundabouts lead to speeding problems  Approach speed should be about 30mph

Capacity Determinants (British)

Capacity is a function of six geometric parameters
Signalised Junctions
Inscribed
circle
diameter, D
Entry width, e

Approach half width, v

Effective flare length, l’
Entry angle,
Entry radius, r
2

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Operation Traffic signal control - advantages

 Traffic signals at road intersections allow vehicle movements
to be controlled by allocating time intervals, during which  minimising the space required, particularly at constrained sites where
separate traffic demands for each arm of the intersection can physical restrictions could make other types of control costly and
difficult to provide;
make use of the available road–space.  the flexibility to assist traffic using specific approach arms or
particular categories of road–user and to respond to a wide range of
different traffic conditions;
 the ability to make special provision for pedestrians and cyclists;
 the ability to link and co–ordinate with other adjacent signal–
controlled junctions to influence the pattern and speed of traffic
progression; and q relatively low cost since, for example, capital costs
are usually less than for conventional roundabouts or grade–
separation.

Traffic signal control - Disadvantages Rate of

discharg
Saturation flow

 increased delays and operating costs, especially in uncongested

e of
queue in

conditions, such as at off–peak times, when signals may impose

fully
saturate
d green
more delay and operating costs on traffic than is necessary to period

resolve conflicts safely; l1, starting lost l2, end lost

time time
 some increased risk of certain types of traffic accident, such as
front–to–rear collisions under braking; Effective green time

 the maintenance costs of signal equipment, with the additional

requirement continuously to monitor signal operations and to
update signal–settings under fixed time control; and q the limited time

facility for U–turning manoeuvres

Actual signal timings
R R/A K A
R

Effective Green, G Effective signal timings