Saturday 16 October 2010

WX02UNH and critical sections

In computing a critical section is defined as something in which only one entity can have exclusive use of at at time, such as, say, a stretch of road or a single-lane railway track. The different bits of the system need to cooperate to gain access to these areas. One way of co-ordinating this access is the semaphore, a concept from the Dutch Computer scientist, Djikstra, based on the old railway notion of flag waving.

Of course, if a French or Italian person had come up with the idea, they'd have used a different name, like "l'indicateur", the car indicator. Because in these countries, to gain exclusive use of an overtaking area, you put your indicator on -way before you are ready to pull out. In the Alps, to put your indicator on before the turn has finished, before you can see if it is safe to pull out, tells everyone else that you intend to, that you have acquired exclusive use of the oncoming traffic lane.

This is why we have one little criticism of the Corsa WX02UNH on Pembroke Road.

We aren't going to criticise it for overtaking the bus on the wrong side of a traffic island. Yes, you aren't meant to do that, but if the anti-car council is going to conspire with Firstbus to put a bus stop in a traffic island, how else are you going to pass it.

Yes, it may be between 8 and 9 am, peak school run hours, but it is also peak commute hours, so the driver may be in a hurry.

No, what we are going to criticise them for is failing to indicate when they pulled out. They just assumed that nobody else was going to be aggressive and take the overtaking opportunity, when in fact any driver in front or behind could have -and because WX02UNH didn't indicate, they would have no warning that the other car was about to pull out. If two cars had collided while trying to drive the wrong side of a traffic island to overtake a bus -now that would be a complex one for the insurance company.

This is why our driving strategy is "Signal then commit". Your signalling is not a hint "I'd like to pull out", but a warning "Here I come", something people should see. But if you don't do such a signal, you don't help others to get out of your way.

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