when it comes to reducing pollution, just how many cars are sitting in the queues, emitting exhaust smoke at the most uneconomical speed (tickover) in those queues, to gain a pollution reduction for those using the train.
A study was done in Canterbury, they shut the westgate Arch to traffic through the arch (the other direction goes around the side) and diverted it across the level crossing and around the alternate route, as an aid to reducing traffic emmission pollution in the area - the traffic would flow much better (they said)
at the start of the 'trial' they changed the footpaths so that you could not drive through the arch.
After 3 months the 6 month trial was brought to an end, due to the doubling of the traffic exhaust pollution levels in the roads the leading to the level crossing on each side. they had to undo the footway works and new kerbs they had installed (in anticipation of the positive results of the trial) and re-open the archway to traffic (it is now shut to busses and coaches - but then the double decker busses were special models designed to get through the arch*(about 1" to spare) and coaches were always getting stuck half way through.
* the special busses were tapered at the top and the wing mirrors were narrower, if your mirrors cleared either side by 1/2" you were central in the arch and could get through. coach mirrors always hit the sides.
you can see the taper from where the windows start on the top deck in the picture below.