Trams can get through traffic faster than buses or cars without lane segregation due to green wave traffic light pre-emption / dynamic tram priority signalization

Trams can share road space with cars without being delayed – have a look at the above video – you will see the trams streaming along, with no cars in front, but a queue of cars behind….


Here is an explanation from Professor Lesley a transport expert:

“Dave Andrews has asked me to respond to several emails: London Road is like many urban roads throughout the UK (and EU). It performs several functions; long distance through traffic, inward commuting trips and local traffic movements. My guess on the basis of the data available is that about 20% of London Road is through traffic, not stopping in Bath.

The Bath tramway plan is aimed at the other c70% of car movements, with 4 out of 5 having only one occupant. For external commuting trips, edge of city P + R with the tramway would provide an acceptable alternative to a large number. For local car trips, walk and tram ride is also acceptable for a significant number of trips. On other UK tramways over 25% of passengers have left a car at home, and in Croydon traffic volumes have reduced by a fifth, as traffic management measures have been introduced to assist the tramway, and discourage ‘suppressed’ car trips from filling the space vacated by car trips attracted to tram.

Each Bath tram will have a transponder, which will indicate its position to the area traffic (signal) control computer. The computer will have algorithms that will calculate the optimum moment to turn a traffic signal in front of a tram to green to release queuing traffic, so the tram has a junction crossing without delay. The logic is not creating ‘gaps’ every 6mins, but keeping the traffic in front of trams moving with a ‘green wave‘. Part of this is to meter traffic entering from side roads, as on motorway slip roads, so that the main road does keep moving. This was first tested in the 1970’s on the Bitterne Road in Southampton. Car traffic (with av. 1.2 occupants) has no worse delay, except it queues on side roads, rather than the main road.

At a typical traffic signalled junction, the (cycle) time from green to green is usually between 60 and 90 secs. So there will be at least 3 cycles without trams, when the tram green (phase) time can be used by other traffic. It is Government policy that trams should enjoy pre-emption of traffic signals, since without that it has been calculated for the new Preston tramway, that on average there would be 25 minutes passenger delay for each tram crossing a busy junction.

Putting these two approaches together; attracting many external commuting and local trips to tram, coupled with ‘green wave‘ traffic signal control including tram pre-emption, will keep traffic moving on London Road, and similar roads in Bath, so letting trams operate a speedy service (c20 – 25km/hr including stops). For many car trips trams will give a journey faster than driving (including parking time) for some people.  Econometric equations calculate the probability of car trips switching to tram, based on the relative quality of service and cost of travel. There is more about these points in my “Light Rail Developers’ Handbook”.

With kind regards,
Yours sincerely,
Lewis Lesley

Note:  Its generally not practicable or politically viable to apply Green Wave to buses, because a 350 person tram only  requires a preemption every 6 minutes, whereas it would take 5 buses to achieve the same effect, and this would require a preemption every minute, and this would hold up other traffic.

Also the bus would not take off the  25% of the car drivers as the trams have in other British tram installs.  ( car drivers will switch to trams but not buses it has been found)

There are a number of interlocking reasons why trams are faster through traffic than buses or cars:

First: Trams only stop for 20 secs, because they have large multiple doors and off tram ticketing. This is not available with buses which therefore have to pull over for an extended time whilst people queue and pay and which delays and frustrates users. The bus then has to force its way back into the traffic which has passed and queued in front of the bus; whereas the tram stays in the road, holding back cars for only 20 secs which is a minor inconvenience and crucially means that there are no cars in front when it restarts. Another factor is that bus passengers have to sit down, closely packed, due to the violence of a buses movement – in trams due to the gradual acceleration and slowing, most passengers stand, as per the London Underground which greatly speeds boarding and deboarding.

Note 1 – the slightly delayed cars more than make up for this due to the unimpeded run they get behind the tram

Note 2 – First Bus recently found that even with a pre-paid ticket app the fastest time the could load a double decker was 2 mins 30 secs – see the video here

Second: Because trams give a much better service than buses (fast, reliable, frequent, comfortable, prestigious) trams are well used, so traffic authorities will grant them traffic light pre-emption rights which they do not do for little used buses ( due to complaints from motorists about underused buses having these rights). Traffic light pre-emption means that when the traffic computer detects a tram coming, well before the tram arrives at a junction, it can preemptively set ALL lights in such a way as to clear the road ahead before the tram arrives  by a) preventing other cars from joining the road ahead and b) setting the several sets of  lights ahead to green to move the traffic forward; this means the tram gets a clear run when it arrives at a given junction, even without segregated roads space – it’s been done for trams in Zurich since the 70s and most other tram systems. ( but doesn’t work for buses)


Above – simplified video of Green Wave Traffic Light Priority – in fact it would not be applied to just the one junction, but also simultaneously to sufficient of the lights ahead to ensure no impediment to the tram progress, factoring time at stops.

For example imagine a tram approaching the Batheaston bypass / London Road junction from Batheaston – before the tram arrives the lights on the bypass slip roads are set to red to prevent cars entering the roundabout and the London Road from the bypass; also sufficient of the London Road lights ahead are set to green, so that when the tram gets to the roundabout over the bypass there are no cars on the London road, such that it may proceed at its normal speed, allowing for scheduled stops. All timings calculated by a central computer. This can only be done if the tram is on a regular well defined time schedule since the lights as far ahead as the Wellsway Roundabout may need to be set.

Note: Many people may say well let’s apply all this to buses.  For economic reasons that just doesn’t work for a number of reasons:

  • Due to the need for economy buses are cheap and cheerful and don’t last long so its not worth fitting the expensive off tram ticketing arrangements.
  • Cheap and cheerful means they are structurally unable to have large multi- doors.
  • Bus company on tight margins are constantly juggling, changing and withdrawing routes so this does not justify the installation of the complex equipment necessary for logging tram positions and the central control systems.
  • Trams are an expensive long term operation and can support the extra capex needed for smooth running through traffic and off-tram ticketing

The World Bank has this to say about traffic light pre-emption – 1/3 way down the article

“In some cases, public transport vehicles may be given ‘zero wait’ priority – this is usually only done for trams, but may also done for BRT. In this case, the phases are extended or truncated sufficiently quickly that the light is always green when the vehicle arrives. The vehicle detection point needs to be located at an appropriate distance to allow sufficient time for the phase changes to be done safely.”  It neglects to say that side roads are also shut of, and green lights are set ahead of the tram also so that their is a wave of no traffic continuously in front of the tram. ( Prof Lewis better explanation further down)

World Bank Toolkit on Intelligent Transport Systems for Urban Transport

Third: Trams can accelerate and decelerate more quickly than a bus

Lane segregation

Notwithstanding all of the foregoing, there are large parts of the proposed network which could have a dedicated tram track – notably most of the London Road, and most of the Wellsway even close to the station. This would entail a single line down the centre with passing loops – this is done today in the centre of Bilbao for example a modern system and also Lisbon.

Beneficial effect on motorists

This system is in fact beneficial for motorists as it organizes them into “platoons” which can move much faster than in normal traffic, and by virtue of the tram taking cars off the road the remaining drivers have a faster drive.