All entries for Tuesday 25 March 2008

March 25, 2008

Finding the shortest route…

As I was driving down the M6 yesterday evening there was an accident that caused a bit of a hold-up. Not knowing how long this was going to take, I hit the “detour” button on the SatNav to see if there was a convenient way around the blockage. Oddly, after finding an alternative route, the SatNav’s ETA was earlier than previously, even though I have it set to plot the quickest route. If the detour was quicker, why wasn’t it sending me that way in the first place?

It turns out this is a variation of the Travelling Salesman Problem, which is hard to solve both accurately and quickly. I guess the SatNav is using an algorithm that gets close to, but not always exactly, the shortest route. To be fair, the difference was only a couple of minutes. I was just surprised, until I thought about it a bit more…

Motorway lane discipline

The highway code rule 264 says:

You should always drive in the left-hand lane when the road ahead is clear. If you are overtaking a number of slower-moving vehicles, you should return to the left-hand lane as soon as you are safely past. Slow-moving or speed-restricted vehicles should always remain in the left-hand lane of the carriageway unless overtaking. You MUST NOT drive on the hard shoulder except in an emergency or if directed to do so by the police, HA traffic officers in uniform or by signs.

I was driving down the M6 yesterday evening. The traffic was dense enough to keep the average speed down below 70mph, but for large parts of the journey the traffic occupied lanes 2 and 3 (or 2, 3 and 4) leaving the left-hand lane empty. Why? What is it about the left-hand lane that makes people not want to use it? Things would flow much better if all of the road was used, instead of just 2/3 of it.

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