Monday, 4 February 2013

Britannia Bridge - no third bridge necessary.

I see that the option for a third crossing of the Menai Straits is 'back on the agenda' because it can take 15 minutes to move one mile....see Daily Post.

But that's the costly wrong option.

If you want to know the theoretical capacity of a road, you study similar roads, in fact ideally more than one hundred similar roads, otherwise a thing called the T-Student distribution means the results are in effect meaningless. So you find your one hundred similar roads or more and measure them over a certain period of time, not just once but numerous occasions. And then some very clever mathematicians at TRL the Transport Research Laboratory come up with clever models that explains it all.

But it's not that simple, in studies of Pelican Crossings for instance, you know the one where you press the button and the light turns red....what they have found is that on certain roads the incidents of injury or death is actually higher where there is a pelican crossing. That could be down to we assuming because the green man is flashing it's safe to cross and we don't look or because we are old and can't judge distances that well anymore when we are the driver or pedestrian.

So let's be clear whilst the world is a complex place, sometimes a simple answer is the solution. In the pelican crossing case you realise that on certain roads the answer is to replace them with a zebra crossing, so that both pedestrian and vehicle driver share the same risk, they take more care. In other words sometimes the answer you think is right, as in pelican crossings are surely safer than zebra crossings isn't the case in all circumstances.

I specialised in roundabout design, and I found that in order to increase capacity the answer was in most cases a relative cheap tweak of the design. The solution to a complex problem wasn't necessarily a complex or expensive answer.

And let's be honest in terms of the Britannia Bridge where people take 15 minutes to travel a mile - first of all is that congestion compared to the M25, I doubt it is. And to solve the 15 minutes to travel one mile problem we apparently need a new bridge - really we've got the money to build a new bridge when we cant afford to paint our schools.

Now I've suggested the solution before, that is active traffic control, that we slow approach speeds to the bridge - it was in my post last year, Britannia Bridge and active traffic management.

Think of it as a wine bottle, and your pouring yourself a drink, there is no point rushing it, you need a gentle flow...and to do that sometimes you need to slow the speed of wine leaving the bottle- we regulate the flow, the speed. And there within lies the answer to the log jam on Britannia Bridge you need to control the approach speeds - Simples really....

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