simont | (no subject)

There are long-term roadworks on the M25, starting just south of the M4 junction. This means that if you're driving to Reading from Cambridge, as I was this morning, you get stuck in the tailbacks from those roadworks just as you thought you'd survived the worst the M25 had to offer.

After crawling along in this jam for a little way, I saw a sign warning me about it (gee thanks). The sign said ‘Delays possible until Dec 2005’. Whoever chose that wording, I thought, was having far too much fun; meanwhile, better phone my destination and let them know I might be a few months late.

On the way back there was a jam on the M4, in which I crawled along for three miles or so at an average speed of perhaps 15-20mph, and eventually discovered that the entire cause of the congestion was an accident on the other carriageway, at which everybody on my side was slowing down to rubberneck. I imagine this will be old news to many readers, but I found it utterly gobsmacking that a slight glance to one side and perhaps a slacking-off on the accelerator, on the part of the drivers at the front of the queue, can slow down traffic by a factor of three two miles further back. Unless the drivers at the front really are slowing right down to 20mph to get a good look, which I'd find even harder to believe. I want to see a detailed replay of the incident (or one like it) as seen from a passing helicopter. Anybody got one?

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Rubbernecking causes nearly all the tailbacks round here. We really are a nosy lot :-)

I've seen a paper about how to reduce traffic jams which cited a paper that apparently said that yes you can have that effect.

You see, what happens is that the person behind the first rubber-necker breaks slightly harder than they need to to avoid hitting them, and then the next one breaks slightly harder again and so on…

If people actually maintained sensible distances between themselves and the people in front then they'd slow down more sensibly and you'd get negative feedback, rather than positive.

I make it a point to follow further than i "need" to, for precisely this reason. I can coast to handle most of the normal "braking without stopping" scenarios, which is much more efficient for both traffic patterns and my own fuel consumption.

It also gives me room to maneuver in, a little runway to pick up the extra 20mph needed to really blow some jackass's doors off. But that's just a happy consequence of an efficiency-based strategy. I swear.

I do the lots of stopping distance approach on motorways too. This is partly compensation for my (suspected) slower reaction times due to driving infrequently, and partly a response to having read some New Scientist article years ago about the compression wave caused by people driving too closely together and then having to brake suddenly.

Also, I'm usually driving hire cars and I don't want to pay any excess on the shiny cars. I could pay extra to reduce the excess, but I find the prospect of incurring large costs if I'm stupid is a handy motivator (and saves me money every trip so far).

I try to do that too, but it doesn't tend to work well for me because some bugger from another lane always moves into the space I'm leaving...

You can look on that as a good thing, as it allows them to make a lane change without slowing everybody else down even more...

You see, what happens is that the person behind the first rubber-necker breaks slightly harder than they need to to avoid hitting them, and then the next one breaks slightly harder again and so on…

This has a name in traffic management circles, which I've completely forgotten as I'm not doing political work in that area any more.

One of the Police Stop videos actually has a very good example on it, filmed mostly from helicopter.

Oh, I remembered - it's called a compression wave effect or something like that. Makes sense.

Whilst on a business trip in Holland, I noticed that at least some of the major roads there seem to have a screen dividing the carriageways (as well as ones at the edges to reduce the escaping noise in urban areas, I imagine). This seems naively like a rather good idea - there's almost no circumstance in which I can imagine that being able to gawp at the other carriageway is actually helpful or liable to increase your safety. Well, unless perhaps a lorry is about to plough through the central reservation, but the odds of your being able to do something useful in that situation seem rather low in any case...

As usual, the question is not whether it's a good idea to screen off the other carriageway, but whether it's a sufficiently worthwhile idea to justify the cost (see also: better fencing where motorways pass over railway lines)

Er. If you got to the end of the traffic jam and lo, at its end there was an accidient accross from you, do you necessarily know that that was the cause of your congestion? shurely (sic) it depends on how fast the compression wave travels relative to its constituent traffic?

On the occasions when I have been on that particular part of Odegra (chiefly, going home from school) some jams will have an apparently obvious cause, but they can also suddenly clear up for no visible reason.