(See Also: https://bathtrams.uk/how-paradoxically-trams-can-speed-through-traffic-without-having-reserved-lanes-buses-generally-cant/) Green Wave Traffic Light Preemption lets tram move though traffic even though previously the road was congested and there is no segregation.
You can see a video of this working in Brussels here where you can see the tram leading the congestion in.
The World Bank discusses traffic light priority systems here: https://www.ssatp.org/sites/ssatp/files/publications/Toolkits/ITS%20Toolkit%20content/its-applications/traffic-management/traffic-signal-priority.html
Extract from above
- 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. This also means that cars following the tram are not delayed and infact overall this has little effect on the despatch of cars comared to the previous situation.
The reason for this is, that to carry the same number of passengers, as a tram, 4 – 5 times as many individual bus movements would need to be accommodated, and these would create far to many self-defeating pre-emptions. You can see this when the vast majority of londoners moved around with so many buses that they created bus jams ( you need 5 times as many buses as trams because they have a much lower capacity)
Buses crawling along in London, 1950
Furthermore highway authorities are reluctant to give priority to vehicles such as buses not carrying many passengers, or a large proportion of traffic – in Bath for example it is about 8% by bus.
From Professor Lewis Lesley
Liverpool once had a comprehensive network of bus lanes. All gone. Why ? As
Dave points out buses will always be the minority vehicle (except in Oxford
St. London), and in Liverpool (a labour authority) (no doubt elsewhere)
most labour voters drive cars, and complained when they had to queue next to
an empty bus lane, which despite having fewer vehicles, still in the peak
carried more passengers. Added to this there are often a myriad of
operating hours for bus lanes, and kerbside bus lanes make a convenient
parking/unloading space, so enforcement is difficult.
Trams in contrast need access to their lane (track) 24/7, so the middle of
the road is a good space, and DfT policy is for trams to have priority at
least at traffic lights. Trams self enforce.
Professor Lewis Lesley