Junction Design – Important Considerations

A junction is defined as the general area where two or more highways join or cross within which is included the roadway and roadside facilities for traffic movements in that area.

The main aspects of interest to traffic and highway engineers at junctions are:

• Capacity, queues and delays;
• Safety, accidents;
• Environments.

Since a junction involves conflicts between traffic in different directions, its scientific design can control accidents and delay and can lead to orderly movement of traffic. The junctions represent potentially dangerous locations from point of view of traffic safety. It is believed that well over half the fatal and serious road accidents in built-up areas occur at junctions.

Constraints on junction design

Driver behaviour constraints –

• Ability to estimate speed and distance for oncoming or following vehicles;
• Ability to adjust speed to the physical layout of the junction;
• Ability to perceive all necessary information on the geometric form of the junction, road markings and signing in time to act appropriately;
• Ability to select safe gaps for merging and crossing.

Traffic constraints –

• Traffic volume and future growth;
• Seasonal, day to day, hour to hour, directional traffic volume variations;
• Traffic composition, design vehicle;
• Turning movements;
• Pedestrians, cyclists and public transport needs.

Geometric constraints –

• National standards for junction design;
• Number of approach arms;
• Angles between arms of the junction;
• Number of through lanes;
• Number of separate lanes or facilities for motorcyclists, cyclists, pedestrians, public transport;
• Sight distances;
• Distance to next junction(s).

Principles for a good junction design

• The number of junctions should be kept a minimum. If necessary, some minor roads may be connected with each other before joining a major road;
• The geometric layout should be so selected that hazardous movements by drivers are eliminated. This can be achieved by various techniques such as channelizing and staggering;
• The design should permit driver to discern quickly either from the layout or from traffic signs the path he should follow and the actions of merging and diverging. This can be achieved by a good layout, traffic islands, signs and carriageway markings. Good visibility improves safety;
• The layout should follow the natural vehicle paths. Smoothness, in contrast to abrupt and sharp corners, should guide minor streams into stopping or slowing down positions;
• There should be consistency of design along a route, so that drivers know what to expect. Driver expectation means that if possible junction layouts should be as simple as possible, so that they are easily able to see the layout and perceive what is expected of them at the junction in terms of their path through it and controls to be obeyed. Signs and road markings clearly can assist this process;
• The number of conflict points should be minimised by separating some of the many cutting, merging or diverging movements;
• Vehicles that are forced to wait in order to cross a traffic stream should be provided with adequate space at the junctions.

At-grade and grade separated junction

The choice between at-grade and grade-separated junction at a particular site depends upon various factors such as traffic, economy, safety, aesthetics, delay etc. Grade-separated junctions generally are more expensive initially and are justified in certain situations. These are:

• On high type facilities such as expressways, freeways and motorways;
• Certain at-grade junction which have reached the maximum capacity and where it is not possible to improve the capacity further by retaining at-grade crossing;
• At certain locations which have a proven record of bad accident history when functioning at-grade junction;
• At junctions where the traffic volume is heavy and the delays and loss caused justify economically the provision of grade-separation;
• At certain specific topographical situation where it is logical to provide a grade-separated structure rather than an at-grade junction, which may involve considerable earthwork or acquisition of land. 

Forms of control at at-grade junctions

Uncontrolled/implicit priority – suitable for junction of two secondary roads in rural or urban areas. The traffic volume and speed is low. It is not recommended if the junction has more than 4 legs. Channelized layouts normally have priorities indicated by signs or marking.

Major/minor priority – suitable for all rural or urban junctions with not more than 4 legs provided that traffic is not too heavy. The road without priority should be controlled by a ‘yield’ or ‘Stop’ sign. The latter is preferred where visibility is reduced and speeds are high, or where accidents are frequent. The channelized version with special acceleration or deceleration lanes presents the most elaborate layout which ensures a high level of service on the major road.

Off-side priority – suitable for urban and suburban junctions. This form of control prevents the junction from becoming blocked at peak hours. It is mostly used at roundabouts where entering traffic has to give way to circulating traffic. Hence delay may be encountered on entry, but once on the roundabout right of way passage is given to circulating vehicles. This is suitable for junctions with 3 arms or more. Any size of central island can be used. On a major route, this type of junction lowers the level of service. 

Traffic signal – Junction with traffic signals will usually be used only in urban areas. In rural areas, signals should only be used in exceptional cases or where temporary control is needed. This form of control is best suited for crossroads. This is recommended when pedestrians are numerous, visibility is bad and/or accidents are frequent.

Traffic signal at roundabouts – used to improve operations at congested roundabouts, often operating in peak period. TRANSYT used to link signal at different entries.

Safety and accidents considerations

On rural roads there are three main groups of accidents:

• Overtaking;
• Loss of control;
• Junction.

On urban areas vehicle/pedestrian and vehicle/cyclist accidents must be considered.

Accidents at junctions often produce black spots. Are accidents treatable? Most junction accidents are, by alterations to layout or form of control. Because of high speeds producing more severe accidents and relative ease of altering junction in rural areas, these junctions have often been an initial focus of attention.

Major/minor priority and roundabouts are ‘space’ sharing junctions. Whereas traffic signals are ‘time’ sharing. The former type of junctions involve drivers in a gap acceptance decision whereas at traffic signals the drivers merely respond to the signal aspect. For poorly maintained vehicles, with lower braking and acceleration capability a ‘dilemma’ zone may occur at traffic signals at the end of green stage, as the driver is unable to stop in time or accelerate clear of the junction. Here a longer inter-green should be considered.

Intrinsically traffic signals should therefore be safer than other forms, if used in urban areas. In some countries there is not good driver compliance with the signal aspect. In the UK the past has seen very good compliance, but more recently there has been increasing incident of drivers continuing through the junction at the start of the red aspect. To deter this, cameras have been installed at some junctions.

The roundabouts have usually been found the safer form of control, as they are better able to cope with large right turning volumes.

Potential conflicts between vehicle paths at junctions underlie accidents. There are crossing, merging and diverging conflicts.

Reduction in number of conflict points can potentially reduce the accidents to a great extent. In UK, staggering of crossroads has the potential of a 60% reduction in accidents.

Visibility at junctions

The safety of traffic can be ensured only if the visibility is full and unimpeded along both roads. Any obstruction should be clear of the minimum visibility triangle for a height of 1.2m above the roadway.

On junction involving a major road and minor road, it is customary to control traffic on the minor road by STOP or GIVE-WAY sign or/and road markings. Such junctions are known as ‘Priority Junctions’. On priority junctions, the visibility provided should be such that drivers approaching from the minor road are able to see the vehicles on the major road in good time and indulge whether the required gap is available in the main road traffic stream for a safe crossing. For this purpose, a visibility distance of 9-15m along the minor road is considered fine.

On the major road of priority junctions, the visibility needed along the major road depends upon the time required by the drivers on the minor road to perceive the traffic conditions on the junction, evaluate the gaps in the vehicle stream, take a decision about the actual crossing and finally accelerate the vehicle to complete the manoeuvre. A time of 8 seconds may be considered appropriate for this purpose.

Speed change lanes (acceleration/deceleration lanes)

They are warranted on high speed, high volume roads from accident and delay considerations. They should be of sufficient width and length to enable a driver to manoeuvre vehicle onto it and then reduce or raise speed to that of the turning roadway or through traffic stream.

Median shelters

On dual carriageway roads at the design stage it is beneficial to consider widening the medians, not only to incorporate any offside diverging lane on the major road, but also to provide shelter for right turning vehicles from the minor roads, so that the gap acceptance decision is split into two easier stages.

Corner islands

These gives turning guidance to drivers and are often used in conjunction with speed change lanes. If corner island area > 4.5 sq.m, then the island should be raised. Avoid large islands delineated only by pavement markings. Corner islands assist pedestrians in crossing wide areas of pavement at busy junctions.

Corner radii

On curves, the rear wheels of vehicles track inside the front ones. On open highway curves the effect isles pronounced and for a 3.65m lane width, in UK pavement widening is only required on curves of radius < 150m. On intersection curves, where generally radii are smaller, the effect is more pronounced.

An appropriate design vehicle should be chosen (e.g. a 15.5m articulated vehicle on collector or arterial roads in the UK) and its track width on different turning radii and lock (steering angle) determined. Templates have been used in the past based on these track widths to determine suitable corner radii. Computer programmes, such as Auto Track etc. are available simulate turning vehicles at junction.

Not only should adequate corner radii be provided, but the design vehicle track width should be used to ensure that turning paths from the major road do not conflict with waiting vehicles on the minor road.

Channelization of junction

A channelized junction is one in which traffic is directed into definite paths by islands and markings. Channelization serves the following purposes:

• Separation of conflicts;
• Control of angle of conflict;
• Control of speed;
• Protection of traffic for vehicle leaving or crossing the main traffic stream;
• Protection of pedestrians;
• Elimination of excessive junction areas;
• Blockage of prohibited movements;
• Location of traffic control devices.