Why Trams Cut Through Traffic But Buses Cannot

This ability is one of the main reasons car drivers will use trams

See also: https://bathtrams.uk/solving-baths-traffic/why-trams-cut-through-traffic-that-buses-cannot/

Trams do not need separate segregated lanes – they use the same road space as cars. This car in Lisbon, below left,  is reversing, normal practice in this city where narrow streets are happily shared with trams.

In Croydon above the trams travel on the same main road as do other vehicles, no special lanes are needed.

Trams don’t get stuck in traffic is because many of the previous car drivers have vanished inside the tram  – because they see it as a quicker, faster and more spacious option.

According to B&NES every day over 73,000 people travel into Bath by car. A modern tram can take 350 people per tram, who could be mainly ex car drivers. With a 6 minute frequency and 11 routes this means that trams could deliver around 40,000 people in one hour, 80,000 people in two hours. This is more than the entire number of people entering Bath each day.

Car drivers find cars attractive because they are spacious, every 6 minutes and reliable. So if they all went in by tram there would be no congestion and the trams would run in free – this is clearly unrealistic, but obviously trams will take a huge number of car drivers off the roads making free space for the tram.  This is what happens in other cities.

Put another way, a tram uses about 1/80th of the road space of the queue of cars that the passengers would take up if they had chosen to drive.

With the reduced numbers of cars because previous drivers are now in the tram, it is possible to apply  “Green Wave” Traffic Light Pre-emption.

“Green Wave” Traffic Light Pre-emption

The above video is not in fact quite how the situation would be addressed in Bath – the video has the tram on a dedicated route whereas in Bath the tram will share the single line road space with cars.

In the present situation with all the incoming roads jammed with cars it makes no difference to a bus caught in the traffic if it can only turn the lights green when it arrives at the light because it has already had to queue through the slow moving car stream.

With trams the situation is different. First of all, each tram can take up to 350 car drivers off the road, meaning there is a large space created in the road so the tram itself is less inhibited by traffic. But secondly, due to the large multiple doors and off-tram ticketing, boarding can be as short as 10 seconds.  This means the tram stops in the road and does not have to pull over like a bus which has much longer boarding times as people queue through the single door and pay the driver.

So when the tram has picked up passengers, the road space in front has not been filled with cars, as happens with the bus and the tram does not have to fight its way back into the stream of cars.

Another simulation traffic light preemption


When say the tram from Bathford approaches the London Road at the A46 junction roundabout, sensors linked to a central computer will have cleared any cars from in front of it by setting the lights green and  holding cars on the slip roads by setting lights to red a sufficient number of seconds before the tram arrives.

The computer will also have set the next set along the London Road to green also sufficiently early so that as the tram crosses the A46 the next section of the London Road has cleared of cars. Thus when the tram crosses the A46 the driver sees a clear road ahead with any cars in front moving through the next light.  As soon as the tram, plus any following bus passes the A46, the lights revert to their normal cycle.

All other lights on the way into the city can also be preemptively set in a similar manner meaning the tram gets a completely clear run. This cannot be done effectively for buses because the traffic stream is more or less continuous.

It’s important to stress that this does not in fact delay the cars – they are already subject to waiting times at traffic light and all the “Green Wave” Traffic Light Pre-emption does is to slightly shift timings so the cars do not experience any extra delay. But for those drivers who will still choose to drive in they will experience a much freer journey because the tram will have taken so many cars off the road.

Another important point is that this means that both rural and local buses can be timed to arrive immediately after the tram so that they can follow it in, so that they are not subject to delay once they arrive at Bath, and this will encourage more drivers to take the busses in because it will be much cheaper and quicker.

As an example of how this might work in Bath see: https://bathtrams.uk/green-wave-traffic-light-pre-emption-work-tram-coming-bathford-via-batheaston/