Reply to post: Re: Car's behaviour makes sense

Oddly enough, when a Tesla accelerates at a barrier, someone dies: Autopilot report lands

Alan Brown Silver badge

Re: Car's behaviour makes sense

"Speed limits mean nothing on California freeways."

60+ years of traffic studies have proven that speed limits don't mean much anywhere. If they're within 5-10 mph of the 'design speed' then drivers cluster to them.

A limit too high for the road will result in most drivers slowing down below the design speed and a limit too low for the road will result in most drivers speeding up to something alittle above the design speed. In both cases "speed spread" will drastically increase (and that's more dangerous than simple speeding as it results in people wanting to pass slower traffic. A slow driver is one of the most dangerous hazards on the road)

The _only_ way to change driving speeds on a road is to change the road and a lot of the changes you'd think are obvious (like narrowing lanes to slow traffic) actually have the opposite effect.

This also applies to the presence of pedestrian fencing, crossings, traffic lights, speed humps and parking restrictions in urban environments - perversely they usually result in _lowered_ pedestrian safety because drivers speed up and exhibit tunnel vision. Chicanes only slow drivers down near the chicane and they accelerate away form them, increasing noise and air pollution in the viciniity, whilst paying less attention to nearby hazards.

One of the easiest and cheapest ways to dramatically slow urban traffic is to remove the centreline and turn off traffic lights - but at the same time you'll find that the traffic moves more smoothly and is less likely to snarl up in peak periods. Human factors (aka traffic psychology) turns up a lot of weird shit and it's only recently been applied to roads vs the 50 years it's been the most important part of aviation safety.

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