Alternative Junction Designs

and why they are superior to existing designs

Introduction

This chapter suggests two alternative junction designs, which offer some advantages over the traditional junction design. Hopefully, these designs will improve the flows of pedestrians and vehicles at existing junctions.

Improved pedestrian signals

This design relates to a standard four-way signalised junction, as is shown in Figure 1. A controller unit attached to the traffic signals follows a set pattern, called a cycle, which is programmed into it. This signal pattern can be very complicated, but at a junction like this one, it is typically fairly simple.

Figure 1: A four-way junction

The signal pattern is broken down into 'stages'. Each stage has the traffic lights set in a particular configuration. A traditional stage sequence is shown in Figure 2.

In stage 1, traffic is allowed to run north-south, by showing a green light for these directions, and a red-light for the east-west directions.

Then in stage 2, traffic is allowed to run east-west by showing a green light for these directions, and showing a red light for the north-south directions.

Finally, in stage 3, if a pedestrian has pressed one of the buttons, then all of the traffic lights are set to red, in order to allow the pedestrians to cross the road. If the button has not been pressed, then this stage is skipped, thereby reducing delays to the traffic.

Then the traffic lights start again with stage 1.

Figure 2: Traditional stage sequence

There are two obvious flaws to this arrangement.

If a pedestrian arrives sometime during stage 1, they have to wait through stage 1 and stage 2. This means that once stage 1 ends, they are likely to risk their luck, and run across the crossing, hoping to get to the other side of the junction before the cars start again. This puts pedestrians at risk.
If a pedestrian presses a button, and then decides to cross the road before the pedestrian stage, then the pedestrian stage will run, and the car drivers will have to wait for no reason. This annoys car drivers.

The solution is to provide two pedestrian stages in the cycle, one after the first traffic stage, and one after the second traffic stage, as is shown in Figure 3. Normally, this means that too much time would be given to the pedestrians, as both stages could run during the cycle. The proposal is that there is a prohibition on running two pedestrian stages sequentially. If a pedestrian stage P1 runs, then P2 cannot, and the pedestrians must wait until then next P1 stage. If the P2 stage runs, then the following P1 stage is skipped, and the pedestrians must wait until the next P2 stage. For the typically low pedestrian flows, this scheme will reduce the delay for the pedestrians, at no cost to the car driver. For higher pedestrian flows, there are no advantages or disadvantages to anyone, although typically pedestrian flows in the UK urban areas are low.

Figure 3: Proposed stage sequence

Additional changes are as follows.

Instead of the 'Wait' command displayed at the pedestrian unit, it will say 'Please Wait', and then 'Thank you'. Pedestrians, like anyone else, are more likely to do what they are asked, if they are asked politely.

The existing button would give the pedestrian the option to cancel their request for a pedestrian stage, if they would prefer to walk across the junction instead. Pressing the button once would request a pedestrian stage, and pressing it again would cancel it.

In Abu Dhabi, some of the pedestrian signals are very innovative. Figure 4 shows two examples of what Abu Dhabi has been experimenting with. The first example shows the 'running man' display. When the animated figure is shown in green, then pedestrians may cross. As time progresses, the animated man walks faster and faster, until a red man is shown. Alternatively, a counter in red is displayed which shows how long is left until the pedestrians may cross, and then a counter in green is displayed which shows how long the pedestrians may cross.

Figure 4: Abu Dhabi pedestrian signals

Restricted Movement Junctions

This design improves the capacity of a T-junction by banning one movement.

Figure 5 shows a typical T-junction, and the movements that road users can make at the junction. The junction can function adequately with 2 signal stages, but will typically use a third extra stage as shown. In the second stage, a filter arrow is displayed on the signal head, so that vehicles turning right into the minor arm of the junction can do so. For a large part of the time, the traffic travelling along the main road is stationary. This causes significant delay.

Figure 5: The operation of a standard T-junction

Figure 6 shows an alternative design. The right turn out of the minor junction (the most difficult manouvere) is banned, and so traffic which wants to turn right will have to turn left and then find somewhere (for example, a roundabout) to U-turn. There is a penalty because these right turns are not obvious. However, one of the major arm directions is continously running, and the other (if the minor arm flows are low enough) can run most of the time. The major arm flows are now handled more effectively. The end result is an overall reduction in congestion at the traffic lights.

Figure 6: The operation of the improved junction

The ultimate expression of the design is the 'Michigan Turn', which is well used in Michiga, USA. In this design, shown in Figures 7 and 8, all left turns (in the UK, right turns) are banned, and vehicles which want to turn left have to proceed ahead or right towards the nearest U-turn facility on the major road. Due to the turning radii of vehicles, this system is characterised by very wide medians along the major road. This arrangement improves safety and reduces congestion.

Figure 7:Michigan turns, Woodward Avenue, Detroit

Figure 8: Turning left using the Michigan Turn

The Federal Highways Agency has produced a PDF booklet showing the details of this kind of junction.

The Compact Michigan Turn

The biggest drawback of the Michigan Turn is the width of the median, On wide urban streets where it could be used, the Michigan Turn has a median which is still to wide. If the road consists currently of three 3.5m lanes in each direction and a 5m median (a total of 26m), a Michigan Turn median of 18.5-30.5m would take up almost all or all of the width of the road, as is shown in Figure 9. That is why the Michigan Turn is only seen in the countryside where there is sufficient spare land for the median - and not in the centre of towns and cities, where there is no spare land.

Figure 9: The width of a traditional Michigan Turn median

A new approach is required.

The new layout starts with the example of three 3.5m lanes in each direction, a short flare to the right and another to the left at the stop line as is shown in Figure 10. Normally, the right turn would be a slip linking the nearside approach lane of one direction to the nearside exit lane of the neighbouring direction.

Figure 10: The width of a traditional Michigan Turn median

The methodology is as follows. The left turn flares are closed to the public, and left-turning traffic must use the U-turns. Traffic signals at the U-turns not only make the U-turns safer, but also enable the median to be reduced in width by about 7m, as the traffic can U-turn into all of the lanes on the opposite side of the road, instead of only the off-side lane - this is shown in Figure 11. The traffic signals at the U-turn are synchronised with the traffic signals at the main junction so that as traffic arrive at the U-turn the traffic lights turn green for them.

Figure 11: The width of a traditional Michigan Turn median

This just leaves the traffic which cannot turn in the width of the road, even with the traffic signals.

The solution is to bring back the left turn flares for these vehicles! There's nothing in the rule book that says that this cannot be done. The reason why the Michigan Turn is so efficient is because the large amount of left-turning traffic is removed. The occassional left-turning bus or lorry is not going to stop the Michigan Turn working. Figure 12 shows how the traffic signals can be manipulated to move left-turning traffic at the main junction.

As in a standard Michigan Turn, the two opposite directions of travel happen at the same time. Here, however, a vehicle wishes to turn left from the western side of the junction. The flow from the eastern side of the junction is cut off, a pause is taken to ensure that all of the cut-off traffic has stopped moving, before the left turn movement is given permission. Throughout, one direction continues to provide through and right-turn movements. The concept is that the first lorry to arrive, wishing to turn left, gets the first go, and the lorries approaching from the other direction will have to wait until the next cycle. If buses are permitted to turn left, then a special stage will be needed for both directions, since the objective is not to delay buses unnecessarily.

Figure 12: Enabling the left-turn facility