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Comment by dijit | Hacker News Reader

Technically not true. There are accidents caused by those who slow unexpectedly because they only just realised there's a speed-trap.

It adds an element of unpredictability to the road.

This almost completely offsets any positive safety gain.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3861844/

Which makes me think it's a cash-grab.

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I have looked at the study you linked, I don't find it very convincing because it is only counting the number of incidents (or MVC), but not the seriousness of the resulting collision.

I would assume that if speed cameras decrease the average speed (which might or might not be true!), even if you have the same number of incidents, you should see a reduction in the seriousness of injuries.

I think that's fair, but if it had been on a nation scale you would have debunked it by saying that it was too broad and would be impossible to control all variables, or that it didn't take into account how $your_country does it.

There are other studies which cite this one and they're linked from the page I posted, I invite you to read them. The data is at best inconclusive that speed-traps are functional at preventing collisions and deaths.

The solution seems to be to put of more of them so people constantly expect their presence. More modern designs are also capable of calculating average speeds over longer stretches of the road.

Yeah - here in the UK most long roadworks segments on motorways have a speed limit of 50mph, with average speed checks. It works remarkably well - I barely ever see anyone speed significantly through these segments, as opposed to segments with periodic cameras where people will speed and then slow down for the cameras.

People slowing because there's a speed camera are the problem, not the camera itself. If they were driving at the stated speed in the first place there'd be no need to suddenly slam the brakes on to avoid a ticket.

That study is along a single road in a single city, and only looks at the absolute number of collisions. As it says, one of the main benefits of lower speeds is that it dramatically decreases kinetic energy - but the study didn't take into account how dangerous those collisions that happened actually were. Number of injuries and fatalities seems like a much better measure of "safety" to me than just number of collisions.

the speed camera is there to protect cyclists and pedestrians, not the driver or his car.

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