Design principles of Intersections
Intersection design is an important task. They need to be designed carefully after considering a number of factors. Some of the main design principles are mentioned below-
i. Uniformity and Simplicity- Intersections must be designed and operated for simplicity and uniformity. The design must keep the capabilities and limitation of drivers, pedestrians and vehicles using intersection. It should be based on a knowledge of what a driver will do rather than what he should do. All the intersection movements should be obvious to the drivers, even if he is a stranger to the area thus maintaining uniformity.
ii. Minimise Conflict Points- any location having merging, diverging or crossing manoeuvres of two vehicles is a potential conflict point. The main objective of the intersection design is to minimise the number and severity of potential conflicts between cars, buses, trucks, bicycles and pedestrians and whenever possible, these should be separated. This can be done by:
- Space separation- by access control islands through channelizing
- Time separation- by traffic signals on waiting lanes
iii. Safety- The safety of a particular design can best be assessed by studying the frequency with which types of accidents occur at a particular type of intersection and its correlation with volume and type of traffic.
- Provision for vulnerable road users: Pedestrians(including specially abled) often need to cross a road in two separate manoeuvres. Properly sighted traffic islands have the added advantage that they can be used as refuges by these vulnerable road users especially at intersections on wide roads.
- Provision of good safe locations for the installation of traffic control devices: The possible use of traffic control devices should always be considered; for instance, the design of an intersection to be eventually controlled by signals may differ from one requiring channelization and signs.
iv. Alignment and Profile- The intersecting roads shall meet at or nearly at right angle. However, angles above 60° do not warrant realignment. Intersection on sharp curves should be avoided because the super elevation and widening of pavement complicates the design. Grades in excess of 3 percent should, therefore, be avoided on intersections while those in excess of 6 percent should not be allowed.
v. Encourage low vehicle speeds on the approaches to right-angle intersections- Minor road vehicles intending to cut across major road traffic should approach the intersection slowly so that they can easily stop and give way to through traffic. This can achieved by funnelling by traffic islands, chicanes etc.
vi. Favour high priority traffic movements- The operating characteristics and layout of an intersection should deliberately favour the intended high-priority movements. This principle, principle generally improves intersection capacity as well as safety.
vii. Discourage undesirable traffic movements-Traffic islands and corner radii can be used to discourage motorists from taking undesirable travel paths, and encourage them to take defined ones.
viii. Provide reference markers for road users- Drivers should be provided with appropriate references at intersections, e.g. Stop/Give Way lines which indicate where, say, the lead vehicle in a minor road traffic stream should stop until a suitable entry gap appears in the main road stream.
ix. Provide advance warning for change- Drivers should never be suddenly faced with unexpected. Advance signing that warns of intersection ahead should be provided on minor roads leading to controlled intersections, on all roads where visibility is restricted prior to an intersection, and on high speed roads where it is desirable to cause vehicles to slow.
x. Illuminate intersections wherever possible- Priority for lighting a night should be given to intersections with heavy pedestrian flows and/or with heavy vehicular flows, at roundabouts and where raised channelization islands intrude on what might be considered the ‘natural’ vehicle pathways , and where an interesting road already has lighting.
Other Design Principles
- Design vehicle The turning capabilities of the design vehicles influence the shape of kerb lines and traffic islands, as well as the width of the carriageway, at at-grade intersections, because the off-tracking of the rear wheels of large vehicles require larger corner radii and extra lane widths to enable these vehicles to negotiate intersections easily and safely without having to stop and carry out complicated manoeuvres.
- Parking – An important consideration in urban locales is whether parking is to be permitted on the intersecting streets. If the parking is allowed, it may be appropriate to use a smaller corner radius as the wheel tracks on the approach arm will usually be further from the kerb.
- Approach alignment: The alignment prior to and at an intersection should enable the intersection to be seen and appreciated by the approaching driver. The optimum locations for intersections occur in gentle sag curves or on sections of straight level roads where there is good forward availability. The available sight distance should never be less than the desirable minimum safe stopping distance for the approach speed.
- Auxiliary lanes: auxiliary lanes are often provided at at-grade intersections to improve their safety and capacity. They are normally not necessary at intersections where traffic volumes and speeds are low, e.g. in urban residential areas or on minor rural roads.
- Sight distance- Adequate visibility in both the horizontal and vertical planes is fundamental to safe intersection design. Thus drivers of vehicles approaching an intersection along a minor road should have unobstructed views to the left and right along the major road for a distance( at least equal to the safe stopping sight distance) which is dependent upon the speed of traffic on the main road, so that they may judge when it is safe to merge with or cross the traffic in the near side lanes of the major road.