Friday, 1 October 2010

The Wheels On The Bus Don't Go Round And Round

This is a guest post by Tom of BorisWatch and Stable and Principled.


The news that Transport Secretary Phil 'Petrolhead' Hammond announced joyously to the Sun (and the Telegraph), but let's concentrate on the Sun here, his spin team didn't pick that paper out of a hat, after all) is both confirmation that the worst tendencies of tabloid-pleasing populism survive and indeed flourish post New Labour. More pertinently to myself, it screws up my commute. Here's an explanaion of why the insanity exists purely between the eras of Phil Hammond and Tom Newton Dunn.

I live basically at Junction 1 on the M4, and work basically at office parks in the Thames Valley corridor, where an awful lot of tech firms set up in recent years due to the proximity of Heathrow and the availability of the kind of big shed architecture you need for open plan offices, warehousing, call centres etc. I thus get to know the M4 rather too well, and in one direction it's actually not too bad, since I'm going out of town in the morning against the flow. Coming back in, it's a different story.

First, a history lesson: the M4 into London has always been three lanes until you get to the Piccadilly Line bridge between Boston Manor and Osterley stations, where it shrinks down to two lanes along the notorious 'M4 elevated section' that occurs with awesome regularity on radio traffic reports. There's a reason for this: the viaduct has no hard shoulders, sharp bends and is rather narrow, which is why it is subject to a 40mph speed limit. Great to pretend you're in the USA for a bit, but as a piece of 21st century road engineering, not so good. Also, it's under constant repair underneath to stop the concrete falling off, which interferes with traffic on the A4. This has been the case since 1973 when all prospects of widening the M4 into London vanished due, ironically, to the activities of grassroots 'Big Society'-style groups in Chiswick and Barnes.

In traffic engineering terms, the capacity of the road is governed not by the 'insane' bus lane but by the capacity of the narrow, twisting elevated section with its two lanes dating from the early 1960. The genius of the M4 bus lane (and it is genius, not insanity, although the Sun is always going to have trouble working this out) is that someone realised that the third lane is therefore essentially redundant tarmac. Here's why:

At peak times the capacity of the elevated section is never going to be enough, so you'll always have jams (and forget widening it, which would cost an genuinely insane amount of money, not least because Thames Valley University and GlaxoSmithKline's expensive new buildings now bracket the viaduct at Boston Manor). Therefore all the third lane ever did was provide people with somewhere to park and emit fumes across Osterley Park, while the merge from three lanes to two actually ate road capacity because the process is inefficient due to being run by the autonomous decisions of people in imperfect communication with each other.

At offpeak times the capacity of the elevated section is enough for the traffic, but crucially this means you don't need the third lane anyway (which is why the bus lane is 24 hours rather than peak hours only).

What the bus lane does is combine the merge from three to two lanes with the lane drop at the previous junction, which has the effect of not eating road capacity at the pinch point*. There was also a reduction in the speed limit from 70mph to 50, increased in 2002 to 60, along with allowing taxis to use the lane.

What was the result of the bus lane's introduction? Well, the TRL report on the scheme (PDF) showed precisely what you'd expect - offpeak journey times (uncontrained by the capacity problems of the elevated section) increased as a result of the speed limit, while peak journey times decreased by an average three minutes due to the removal of the merge - at peak times the traffic rarely gets near 60mph, so the reduced speed limit has no effect. At weekends the lower traffic volumes result in the speed limit becoming the limiting factor again, resulting in slower journey times. Overall, the peak hour reliability improvement more than cancels this out, however.

What, then, is the effect of removal? Well, unless they change the speed limit too the weekends and offpeak journeys will be the same as at present, while with the merge restored to the Piccadilly Line bridge, the peak journey times will extend and become less reliable as the road won't be able to cope as well with perturbations due to the loss of capacity at this point. This, in fact, is precisely what you'd do if you wanted to declare war on the motorist and make my life more miserable.

If you actually wanted to improve matters for motorists you could increase the speed limit and remove the third lane completely. This would result in a dangerous drop in speed limit at the bridge from 70mph to 40mph where the Porsche set (travelling at 90) would be perenially rear-ending people, however, so a graduated change in the limit from 70 to 60 to 40 would be required. Alternatively, the best thing to do for motorists, as with anywhere else in the South East of England, is to encourage them to stop driving. Coaches would be good. You could even give them their own lane.

* Actually, this isn't quite true - you still lose capacity due, ironically, to buses and taxis in the bus lane - this was painfully evident the other day when a row of posh Addison Lee limos using the bus lane, I suspect illegally, forced the two normal lanes including myself to a halt at the merge. The bus lane works best the fewer vehicles there are in it, an insight that you really need to grasp, along with the primary importance of the elevated section, before I'll take you seriously on this. It's therefore also crucial that it's enforced properly, which certainly hasn't been the case recently.

1 comment:

Unknown said...

Thank you for this. I've been dumfounded at the number of people who use the M4 who cannot realise the negative affect the removal of the bus lane will have.

I call it the Jeremy Clarkson effect.