How Green Wave traffic light pre-emption would work

How Green Wave traffic light pre-emption would work for a tram coming in from Bathford via Batheaston.

Note –  this is meant as an example to show the principle – how it might work in practice may be different

Green Wave Traffic Light pre-emption means using a central self-learning traffic control computer to set traffic lights pre-emptively, such that a tram can proceed through rush hour traffic unimpeded. This does not mean turning lights green when the tram arrives.

Green Wave distance – this is the distance ahead of a tram that the “last car” in the traffic queue ahead has to be so that the tram is unimpeded by traffic during its journey..

Assume we wish the tram to travel from the Batheaston roundabout on the bypass unrestricted by traffic until it arrives at say the Bathwick Street  traffic lights, a distance of 1 mile, then the calculation of the Green Wave distance is quite easy:

Assume average speed of traffic (at rush hour) is about 10 miles per hour ( Google maps indicates 5 – 14 m/h)  (The Green Wave computer would continuously adjust the calculation below according to actual traffic speeds.)

Average tram speed is 25 km/h including stops = 15 m/h ( which it is in Croydon for the on-street  running portion)

The control computer will  pre-emptively adjust the lights as below so that well before the tram approaches the A46 roundabout from Batheaston, the Green Wave traffic light control computer has set:

  • the slip roads lights off the A46 to red,
  • the London Road and other lights onwards to Bathwick Street  traffic lights to green,
  • all side roads lights onto the London road are at red from the roundabout  up to the “last car” in the queue.

This will mean that there is a clear space ahead of the tram from the roundabout to  “the last car” in the traffic queue.

We want the computer to set these lights so that when the tram actually catches up with the “last car” they will both have reached the Bathwick Street traffic lights which is 1 mile from the roundabout on the A46 simultaneously.

If this is the case because there is no traffic in front of the tram it will proceed at 15m/h and take 4 minutes to get to the Bathwick Street traffic lights.

But in 4 minutes, the “last car” at 10 miles/h will have travelled 4/60 x 10 =  0.67 mile, therefore the pre-emption computer needs to have set the lights as mentioned earlier so that just as the tram arrives at the roundabout, the last car is 1 – 0.67 miles = 0.33 miles from the Roundabout.

For this length of road. 0.33 miles to have cleared from the roundabout, then since the “last car” is moving at 10 mph,  the traffic light pre-emption ie setting green ahead and setting side roads to red, should have started at the roundabout  0.33 x 60/10 = 2 minutes before the tram arrives.

The important point to note here is that this will not delay cars. Instead of slowly moving in the London road traffic,  they will be pausing on the bypass, and the other the side roads. Once the tram  has passed they will follow the tram in, and arrive at the Bathwick Street  lights, just in time to catch up with the last car, which is where they would have been anyway without the pre-emption, so they will be no worse off than before.

Once the tram has passed, normal traffic light sequences will carry on as before for 4 minutes until the next tram is due.

This ability for trams to move quickly into the city will make them attractive for drivers from Bathford / Batheaston, and the proposed park and ride built above the dual carriageway, and this will take about 25% of the cars off the road (from other UK tram install’s experience) thus the actual situation will have improved for not only the tram passengers but also those who still wish to drive, since there will be less cars than before.

We have assumed in this example for simplicity that there are no stops between the roundabout and the Bathwick lights, however it would be necessary to have perhaps a couple of stops, and these would be located conveniently at traffic lights – one obvious one being the Morrisons Supermarket and another at the St Savours Road junction, which would permit the pre-emption time to be reduced from 4 minutes, but this is a question of detailed design – the above example is to explain the principle.

It is also clear why this cannot be applied to buses.  For this to work for buses, because you need three times as many buses, you would need three times as many pre-emptions and this would confer very little advantage since it would essentially be the same traffic light sequences we have now; furthermore there would not be the shift from cars to trams.