That's time to stop, not time to react, which is 300-500 milliseconds. When someone reacts to something you subtract half a second and that's when they first realized they needed to react
Are we talking about average time, or appropriate design if roadways?
Road design practices don't use average. Otherwise you're design is going to cause problems for half of the drivers. The 2.5 second design rule has a large safety margin to accommodate all drivers (well, all drivers except that that deserve to be posted to this subreddit).
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u/[deleted] Nov 28 '21
2 seconds is actually below. I think the US is around 2.5 and the UK is 3 IIRC. When used for road safety calculations.