7. Intersections

7.1 Importance of Intersections in the Road System

An intersection is a location at which two different traffic streams cross or merge.

Intersections are an inevitable part of any road system. Where a significant number of intersections occur they are the major determinants of traffic flow and capacity of the entire road system. They are also the source of significant accident risk.

The two basic types of intersections are

• at-grade intersections, and
• grade-separated intersections.

7.2 Types of Conflicting Manoeuvres

There are three basic types of at-grade intersection manoeuvres:

• diverging,
• merging, and
• crossing.

A fourth basic manoeuvre, the weave, may also be defined, although this is really a merge follow by a diverge.

An elemental manoeuvre occurs when any two one-way, single lane movements interact. A multiple manoeuvre occurs when more than two one-way single lane movements take place. Multiple manoeuvres should be avoided as they confuse drivers, reduce safety and frequently reduce capacity. Intersection design and redesign often involves trying to replace multiple manoeuvres with a series of elemental ones.

7.3 Types of Intersections At-Grade

An at-grade intersection occurs where roads meet or intersect at the same level. Three basic forms may be identified:

• Unchannelised Unflared Intersections. This type of intersection is used where minor roads intersect and where a minor road joins a major road. Many intersections in urban areas are of this type.
• Unchannelised and Flared Intersections. Simple unchannelised intersections may be flared to provide additional through lanes or turning lanes.
• Channelised Intersections. A channelised intersections is one where paths of travel for various movements are delineated by traffic islands, raised markers, painted markings, etc. A roundabout is a channelised intersections where traffic moves one way around a central island.

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7.4 Factors Influencing the Design of At-Grade Intersections

At-grade intersections present a driver with several points of potential conflict with other road users. The general aims of intersection design are to achieve efficient traffic operation and to minimise the risk of collision. This is achieved by controlling vehicle manoeuvres and minimising the number of points of conflict.

The principle factors influencing the design of an intersection are:

• Traffic. An intersection should accommodate all road users with comfort and safety.
• Topography and Environment. The alignment and grade of the approach roads will influence intersection design, as will drainage requirements, location of public utilities and property access.
• Economic Considerations. Alternate intersection designs can be considered economically by cost-benefit studies.
• Human Factors A driver must be aware of the presence of an intersection, as well as the presence of other vehicles and road users within the intersection.

7.5 Design Procedure

The following provides a general approach to at-grade intersection design:

• Step 1. Obtain and analyse traffic data to determine design hour volumes for all through and turning movements including further growth.
• Step 2. Obtain physical data for site e.g. topography, buildings etc. (include future development proposals).
• Step 3. Obtain location, type and general design features of all existing and proposal roads.
• Step 4. Prepare study sketches for several likely intersection schemes that are suitable to meet traffic needs and are practical for the site and design controls.
• Step 5. Analyse alternate schemes and select the best two or more for further study and for the preparation of preliminary plans and profiles.
• Step 6. Prepare preliminary plans.
• Step 7. Evaluate preliminary plans with respect to:
• design features;
• capacity;
• operational characteristics;
• overall adaptability;
• maintenance of traffic during construction; and
• suitability to stage construction
• Step 8. Calculate preliminary cost estimates including:
• land acquisition;
• clearing the site;
• construction;
• maintenance; and
• utility changes. etc.
• Step 9. Calculate road user costs and road user benefits for the alternatives.
• Step 10. Analyse data from steps (7), (8) and (9) to reach a conclusion as to the preferred plan.
• Step 11. Prepare final design drawings, specifications and estimates.

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7.6 Principles of Intersection Design

The smallest unit of intersection design is the individual manoeuvre area, and a typical road intersection can be considered as a combination of a number of elemental manoeuvre areas. To a large extent the arrangement of manoeuvre areas is governed by economic and environmental considerations. The proper compromise between these competing factors will be made by the individual designer.

Intersection design should consider the following twelve fundamental principles:

• Minimise the Number of Points of Conflict
• Control Relative Speed
• Separate Points of Conflict
• Give Priority to Major Traffic Movements
• Reduce Area of Conflict
• Coordinate Intersection Design and Traffic Control Considerations
• Clearly Define Vehicle Paths and Conflict Areas
• Control the Speed and Angle of Conflicting Traffic Movements
• Prohibit Undesirable or Unnecessary Movements
• Provide Adequate Capacity
• Consider the Spacing of Intersections
• Consider the Needs of All Road Users

7.7 Geometric Design Standards for At-Grade Intersections

7.7.1 Design Vehicles

Intersection design should make provision for vehicles of legal dimensions to perform movements with adequate clearance to road furniture and other vehicles. In some cases allowance may have to be made for vehicles which exceed legal dimensions. Three ‘design vehicles’ are normally used in intersection design. These are the design car, the design single unit truck or bus, and the design semi-trailer. The selection of the appropriate design vehicle for a particular intersection depends on the location of the intersection and the expected traffic composition.

7.7.2 Sight Distance

The provision of adequate sight distance at an intersection is fundamental to safe intersection design. Three sight distance criteria are applicable to vehicle operation at an intersection:

• Approach Sight Distance (ASD) This is the minimum sight distance to be provided to a driver to create awareness of intersection layout, etc. and to allow sufficient time to react and stop if necessary.
• Entering Sight Distance (ESD) This is the sight distance required for a driver on a minor road to enter a major road with a left or right turn such that traffic on the major road will not be impeded.
• Safe Intersection Sight Distance (SISD) This provides sufficient distance for a driver on the major road of an intersection to observe a vehicle from a minor road approaching, and to stop before reaching collision point.

7.7.3 Horizontal and Vertical Alignment

The best locations for intersections are on straight roads having a uniform grade. At these locations sight distance is maximised and the driving task is easiest. Where a straight alignment cannot be achieved the intersection should be contained within a horizontal curve, so that drives are travelling on the curve before reaching the intersection. Sharp curves or severe changes in alignment within an intersection should be avoided. In rolling terrain intersections should be located in sag curves rather than on or near crest curves where sight distances are restricted.

7.7.4 Channelisation

The design of channelisation for an intersection should be an individual design for that intersection, based on considerations of traffic pattern, traffic volumes, topography, pedestrian movement, parking, and the planned ultimate development of the adjoining land. When considering the islands to be used in a channelised design it is preferable to have a few large islands than a large number of small ones.

7.7.5 Provision for Left-turning Vehicles

Left turning vehicles are usually catered for by simple left turns, left turn lanes and left turn slip lanes. Simple left turns consist of a curve of appropriate radius and would be used where the left turn volume is low. Left turn lanes may be considered at signalised intersections or where left turn volumes are high. A left turn lane with a corner island at the intersection is referred to as a left turn slip lane.

7.7.6 Provision for Right-turning Vehicles

Auxiliary right turn lanes should be considered at all signalised intersections and where median widths or lane widths allow their provision and safety would be improved. The aim of using right turn lanes is usually to improve capacity and/or safety.

7.7.7 Traffic Lane Widths

Ideally the width of traffic lanes on the approaches should be carried through the intersection. However in some urban situations reduction in lane width to 3m, or even lower, can provide an additional traffic lane.

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7.8 Roundabouts

A roundabout is a channelised intersection in which all traffic through the intersection circulates clockwise around a central island. Entering traffic is required to give way to traffic circulating on the roundabout.

Roundabouts can be used to advantage on a wide range of intersections. Roundabouts are particularly suitable for the following situations:

• multi-leg intersections;
• urban arterial road intersections which have a high right turning traffic volume; and
• T,Y or cross-road intersections where the major flow turns through a significant angle.

The safe performance of a roundabout is dependent on the geometric arrangement slowing all traffic to 50km/hr or less. This may be achieved by:

• a central island of suitable size and position;
• providing appropriate deflection of entering traffic; and
• staggering or misaligning the traffic path between entry and exit.

The central island is usually circular. The island should be kerbed and raised slightly above the level of the circulating roadway. The width of the circulating roadway depends on the number of traffic lanes required, the size of the central island, the design vehicle, and the need to cater for large vehicles. The main function of splitter islands is to guide vehicles into the circulating roadway at an appropriate location and angle.

7.9 Grade Separations and Interchanges

A grade-separated intersection occurs when intersecting roads are separated in level to eliminate crossing conflicts. The roads no longer intersect at the same grade (i.e. the intersection is no longer an at-grade intersection) and so the intersections is referred to as being grade separated. If the intersection also allows turning movements from one road to another, then an interchange is formed. An interchange is the highest form of intersection treatment. When fully developed all at-grade crossing is eliminated and interaction between traffic streams takes place by merging, diverging or weaving.

The advantages of grade separation and interchanges are:

• capacity approaches that of normal road sections;
• increased safety and driver comfort;
• design is flexible – they can be adapted to most situations; and
• staged development is often possible.

The disadvantages of grade separations and interchanges are:

• costly;
• large land area required; and
• may be confusing to drivers.

Although each design situation should be individually considered, there are certain standard design configurations which act as a guide:

• T or Trumpet (3 leg). This form uses a single bridge, and all traffic movement except one turn moves through 90°.
• Y (3 leg). This form uses a simple bridge and incorporates one at-grade movement.
• Cloverleaf. This form uses a single bridge. It takes a large land area especially if high design speeds are used. Vehicles turning right have to negotiate a 270° turn and usually have a fairly rapid change in grade.
• Partial Cloverleaf. At times it will not be desirable or possible to construct a full cloverleaf. In these situations a partial cloverleaf may be possible.
• Diamond. This form has the disadvantage of 2 at-grade intersections on the minor road, but uses less land than a cloverleaf.
• Rotary. The rotary interchange can cope with any number of intersecting arms. It uses 2 bridges.

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Page last modified 24 June 2002.