A Bath to Bristol street running tram through the “Saltford bottleneck” will reduce congestion not increase it.

This article shows how a 103 m of tram can remove a line of previously queued cars of over 1.65 km in length and will therefore speed up the remaining car traffic.

Above – provisional indicative map ( awaiting professional re-drawing)

This article Bath Chronicle, Thursday 8th September,  doesn’t quite get it right:



The West of England Combined Authority WECA is proposing a rapid transit between Bath and Bristol roughly following the existing main road. Some have worried that there is a bottleneck at Saltford because it is restricted in width where there is only room for two lanes in both directions.

In fact, it is not a bottleneck for trams, though it is for buses, and a street running tram can easily be introduced without inconveniencing existing car drivers, in fact making essential drivers’ journeys quicker.

This stems from the fact that a 72 m long low floor articulated tram can carry up to 510 people comfortably and experience shows that at least 30% of those passengers will be ex car drivers and about 20% will have transferred from cars previously used on this trip ( See** below)  due to the extra convenience, comfort, immunity from congestion ( to Green Wave, not applicable to buses)  reliability, frequency.

The above shows the significant modal shift that trams can create ie from car to tram, which does not occur significantly for buses

Since such a tram can run every 6 minutes, at peak this means it can carry 5100 people per hour and about 20%**  will be ex car drivers who previously drove this route= 110 per tram or 1100 ex drivers per hour.

The road can only carry maximum of 1000 passengers plus driver per hour,  so we can say that each tram will be replacing the road space of say 110 cars, each with a spacing of say 15m  ( rule of thumb, 1 meter per mph at say 15 mph speed*** ) 15 x 110  = 1650 m or 1.650 km compared to the length of a tram setabout 73m https://en.wikipedia.org/wiki/Types_of_trams   plus say 15 m space at each = 103 m.

Thus 103 m of tram can remove a line of previously queued cars of over 1.65 km in length and will therefore speed up the remaining car traffic.

Furthermore, the tram from the Globe Roundabout the tram can run in the field adjacent to the road rejoining it at the around the 30-mph restriction ( subject to detailed survey), and after Keynsham on the side of the road also where there is room until it reaches the Keynsham bypass where it could 1)  deviate into and through Keynsham, 2) go along the bypass on the verge or 3)  deviate and pass through the new housing estates planned between the bypass and Kelston, or 4) all three depending on outcome of detailed cost and traffic projections*.

Green Wave Traffic Light Pre-emption  means that as soon as the tram starts its journey from Bath, or Keynsham ALL the lights in sequence as required will be set to permit the tram to run uninterrupted through Saltford up to the beginning of the Salford by pass. This will mean briefly preventing cars from joining the road from the Globe to Saltford at around the 30-mph limit Bath side of Saltford, until sufficient road way ahead is clear, such that when the tram catches up to the traffic queue at the roundabout the queue will have just joined the bypass so that the tram is not delayed.  This will only delay the cars queuing temporarily at the30 mph sign, but is time they will more than make up for because they can follow the tram in at 30 mph which is much faster than their previous frustrating stop start crawl through Saltford.

This cannot work with buses because 6 times as many buses (and the expense of the bus driver would be required, and because it has been repeatedly shown that buses do not have any significant mode shift

London bus jam when everyone travelled by bus – circa 1950

From Agatha Christie’s ” The Clocks” Publication date November 1963

Sheila Webb one of the typists picked up her pad and knocked on the door and entered.

Miss Martindale, the proprietor of the typing agency looked up from her desk….

“you’re late back from lunch Miss Webb”

“Sorry Miss Martindale, there was a terrific bus jam”

Miss Martindale retorted

“There is always a terrific bus jam at this time of day – you should allow for it!

Edinburgh bus jam – pre tram.

You can see a video of Green Wave in Brussels here with the tram leading a platoon of cars into Brussels:

  • One view is that the tram should go through Keynsham (High St/Durley Hill), again with green wave traffic signals control.   Anyone wanting an express Bath-Bristol service can take the train.  Tram and MetroWest rail planning must be an integrated exercise, not pursued separately.


Notes on modal shift:



Modal shift ( ie attracting car drivers) is low for buses compared to trams

The chart above is extracted from the below report by David Walmsley, Chartered Member of the ILT and a Member of the CIHT.  BSc and PhD in physics.


Bath Trams,

Nottingham is said to have 6M fewer cars per year on the city’s roads since the tram, against rises elsewhere. As NET has about 18M trips per year, 6M would represent at least 33%, if each car had only one occupant, so the real figure is probably more like 40%, and even more if the increase in car use elsewhere is taken into account. NET was designed with P+R as a major component, with over 5,000 free parking spaces.


Dave Gibson.


**Comments from David Walmsley. Chartered Member of the ILT and a Member of the CIHT.  BSc and PhD in physics.

The PTEG report gives 19% for Croydon (16% drivers and 3% passengers).

It also gives figures for Manchester as 18% peak, 30% off-peak and about 44% weekend. I have done some averaging between the Bury and Altrincham lines here.
For Nottingham, it says 21% of passengers use the 5 park-and-ride sites, and I would expect it to be a bit higher when other car drivers are included.
I would take Lewis’s 25% figure. I suspect the oft-quoted 30% contains a bit of “optimism bias.”
Remember that there are also likely to be 15 to 20% of passengers making a trip they didn’t make before. **My rule of thumb is “half to two-thirds of the tram passengers are former bus passengers, about half the rest transfer from car and the others are new journeys.”
Also remember that these are percentages of the numbers of tram passengers. 25% transfer from car to tram means 25% of the tram passengers, not 25% of the car journeys.
David Walmsley


Comments from Prof. Lewis LEsley:

Professor Lewis Lesley – BSc, AKC, PhD, CEng, FRSA, MICE, FCIT, MTPS) ljslesley@aol.com. Professor Lesley spent bulk of his academic career studying how to get people to use buses rather than cars.

Croydon is actually 25% of tram passengers leaving a car at home, i.e. diverting from car to tram. The figure for all trips in Croydon along the tram corridor is 19%, since many trips including walking are not attractable to tram e.g. too short or too long.
The US Transportation Research Board (Report No. 1221) analysed 40 years of data of rail converted to buses, and a few bus services converted to rail. This study found that an average of 40% or tram passengers had switched from car, either all the way, or from a park and ride stop. The range was 35% to 45%. Of course car ownership in the US is much higher than UK, c. 750 per 1000 population v 450 per 1000 population in UK.
Above – mode shift San Diego Trolley ( = Tram / Street Car / Light Rail) From Professor Lesley.

” 2.2  pteg recently published a review, by consultants Steer Davies Gleave (SDG), of the record of light rail in the UK. The report (“What Light Rail can do for Cities”), found that UK light rail is popular, with all schemes operating at, or near, capacity at peak times. Overall there has been a 52% increase in patronage since 1999 with significant flows being carried outside the rush hours. This increase has come despite significant increases in fares, and at a time when patronage of the bus network outside London has fallen.”

3.2  This helps to explain the high modal shift that UK light rail has achieved, with about 20% of peak hour light rail users having previously travelled by car. At the weekends modal shift can be as high as 50%. Reductions in road traffic of up to 14% after the introduction of tram schemes have been recorded.

4.7  Light rail’s advantages over the bus alternative are reflected in the much higher levels of modal shift that light rail achieves. As set out in para 3.2, peak hour transfer from car to tram is consistently around 20%. This compares with estimates of between 4% and 6.5% resulting from significant improvements to bus corridors. Finally, as the SDG report shows, improvements to bus services (often perceived as potentially temporary) do not have the same catalytic effect on urban regeneration and city image that can be triggered by the tangible and permanent commitment to an area that light rail represents.

6.2  The bus will remain the mainstay of public transport provision in the city regions and new forms of guided, high quality and tram-like bus systems are being pioneered by the PTEs. However light rail has clear advantages on busy corridors where its greater capacity, speed, quality and reliability have led to far higher levels of modal shift than improvements to bus services have hitherto come close to achieving.

See also “What light rail can do for cities” by Steer Davies Gleave for PTEG (now the Urban Transport Group) and published in 2005.

You can get the full report off the UTG website:


When you say car users will not use buses, that is OK as a generalisation. However, if you have a properly designed  BRT/BHNS service with special livery, designated vehicles, a simple easily understood route, publicity, proper interchanges, through ticketing and – most importantly – a reasonable amount of segregated track, it can work.  Basically, that’s all the things that come as part of the package with a tram, but with a bus you have to work hard to get it right. It also helps to put some restraints on car use in place (pedestrianisation, ULEZ, parking restrictions etc). Where there is a good public transport alternative, car reduction measures  become  more acceptable. And don’t expect it to come cheap; you won’t get 80 per cent of the benefits of a tram for 20 per cent of the cost. Putting an M in front of the route number and calling it a metro doesn’t cut the mustard.


When I was in Bristol about 18 months ago, I decided to try the Metrobus route. It was a failure because I couldn’t find where the bus went from in the city centre. I expect some of your Bristol members could tell me where to find the bus stop, but that’s not the point – if it isn’t obvious, it isn’t working.


David Walmsley  Chartered Member of the ILT and a Member of the CIHT.  BSc and PhD in physics.

*** https://www.drivingtestsuccess.com/blog/safe-separation-distance