Roundabouts, priority junctions (simple T-junctions, staggered T-junctions or crossroads) and signal-controlled intersections are the three main types of highway intersections. These are features created on the highways to control conflicting and merging streams of traffic so that delay is minimized. Presently, flyover bridges are also used as a means to control conflicting traffic when there is a very high traffic volume. Within these features, matching geometric parameters are also selected to regulate the vehicle.
Not all types of highway intersections are suitable for a road. With the ultimate aim to provide drivers with a road layout that will minimize confusion as well as enhance flexibility, the most suitable type of intersection should be selected for each road while also considering factors such as economic, environmental, and operational effects of each choice.
Among the three types of intersections, different combinations of turning movements should be tested in order to check the performance characteristics of each junction option under consideration.
Types of Highway Intersections
A roundabout is provided on the road to control merging and conflicting traffic flows at an intersection. A roundabout performs the following two functions:
- It defines the priority between traffic streams entering the junction, usually on the basis that traffic wanting to join the circulatory flow must give way to the traffic to their right already circulating in the roundabout (this depends on the direction that the traffic circulates).
- It causes the diversion of traffic from its preferred straight-line path, requiring drivers to slow down as they enter the junction.
A roundabout must have sufficient gaps to allow circulating flows. It operates similarly to the priority junction; however, its design is more complex because it does not have any identifiable major traffic flow that can be used as a basis for designing the junction. Roundabouts or signal-controlled junctions are more economical options at flow levels above those suitable for priority control. However, if the cost of land is an important factor, then a signal-controlled junction would be the most viable option because the cost of a 3 or 4-arm conventional roundabout would be greater.
Roundabouts are also more suitable in situations where there are:
- A significant change in road classification/type.
- A major alteration in the direction of the road.
- A change from an urban to a rural environment.
Types of Roundabouts
Mini-roundabouts consist of a 1-way circulatory carriageway around a reflectorised, flush/slightly raised circular island less than 4 m in diameter which can be overrun with ease by the wheels of heavy vehicles. This type of roundabout can be extremely successful in improving existing urban junctions where side road delay and safety are a concern. Mini-roundabouts are used predominantly in urban areas with speed limits not exceeding 48 km/h (50 mph). They are never used on highways with high-speed limits.
In situations where physical deflection of vehicle paths to the left may be difficult to achieve, road markings should be employed in order to induce some vehicle deflection/speed reduction. If sufficient vehicle deflection cannot be achieved, the speed of the traffic on the approach roads can be reduced using traffic calming techniques. Drivers arriving at the intersection of mini-roundabouts must monitor very closely the movements of other vehicles both within the junction and on the approaches in order to be in a position to react very quickly when a gap occurs.
A normal roundabout is defined as a roundabout having a 1-way circulatory carriageway around a kerbed central island at least 4 m in diameter, with an inscribed circle diameter (ICD) of at least 28 m and with flared approaches to allow for multiple vehicle entries. The number of recommended entry arms is either three or four. If the number is above four, the roundabout becomes larger with the probability that higher circulatory speeds will be generated. In such situations, double roundabouts may provide a solution.
A double roundabout can be defined as an individual junction with two normal/mini-roundabouts either contiguous or connected by a central link road or kerbed island. Where a double roundabout connects two mini-roundabouts, the speed limit on the approaches must not exceed 48 km/h (30 mph). A double roundabout is more suitable in the following situations:
- When there is a need to improve an existing staggered junction to ensure it avoids the need to re-align one of the approach roads.
- When there is a need to join two parallel routes separated by a watercourse, railway, or motorway.
- When there is a need to connect two separated opposing right-turning movements at existing crossroads intersections.
- When there is a need to cater for junctions with more than four entries and overloaded single roundabouts overall capacity can be increased by reducing the circulating flow travelling past critically important entry points.
Other forms of roundabouts are two-bridge roundabouts, dumbbell roundabouts, ring junctions, and signal-controlled roundabouts.
A priority intersection occurs between two roads, one termed the ‘major’ road and the other the ‘minor’ road. The major road is the one assigned a permanent priority of traffic movement over that of the minor road. The minor road must give priority to the major road with traffic from it only entering the major road when appropriate gaps appear. The principal advantage of this type of junction is that the traffic on the major route is not delayed.
Geometric Features Suitable for Priority Junctions
1. Horizontal Alignment
In the ideal situation, the priority intersection should not be sited where the major road is on a sharp curve. Where this is unavoidable, it is preferable that the T-junction is located with the minor junction on the outside of the curve.
2. Vertical Alignment
The preferred location for a priority intersection is on level terrain, or where the gradient of the approach roads does not exceed an uphill or downhill gradient of 2%. Downhill gradients greater than this figure induce excessively high speeds, while uphill approaches prevent the drivers from appreciating the layout of the junction.
Along the major and minor roads, approaching traffic should be able to see the minor road entry from a distance equal to the desirable minimum sight-stopping distance (SSD). The required SSD depends on the chosen design speed and varies from 70 m for a design speed of 50 km/h to 295 m for 120 km/h. In addition, on the minor road, from a distance ‘x’ metres back along the centre line of the road, measured from a continuation of the line of the nearside edge of the running carriageway of the major road, the approaching driver should be able to see clearly points on the left and right on the nearside edge of the major road running carriageway a distance ‘y’ away. The ‘x’ value is set at 9 m. The ‘y’ value varies depending on the chosen design speed along the major road (see Table 1).
Table 1: ‘y’ visibility distances from a minor road
|Design speed along major road (km/h)||y distance (m)|
4. Dedicated lane on the major roads for right-turning vehicles
In the case of all non-simple junctions, provision of a right-turning lane, the lane itself should not be less than 3 m. It requires the provision of a turning length, a deceleration length, and a direct taper length. The turning length (a) is set at 10 m; the deceleration length (b) varies from 25 m to 110 m depending on the design speed on the major route. The taper (c) varies from 1 in 20 to 1 in 30 for ghost island/single lane dualling intersections. (Greater values are required for dual carriageway intersections.
Types of Priority Junction (pictorial views)
Signal-controlled junctions are simply priority junctions where traffic lights are installed to be used to control traffic. It works on the basis of allocating separate time periods to conflicting traffic movements at a highway intersection so that the available carriageway space is utilised as efficiently and safely as possible.
A comprehensible article on the design of signal-controlled junctions can be found HERE.
Highway Engineering by Martin Rogers