Here is an explanation from Professor Lewis Lesley a transport expert:
Professor Lesley has studied public transport extensively and has concluded that car drivers will not generally use buses, but they are very likely to use trams for a variety of reasons.
“Dave Andrews has asked me to respond to several emails and articles on the Bath Trams Website
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,
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)