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Comment by baha_man | Hacker News Reader

"However, there is one rule you shouldn't break... the 'three-second rule' for following distances; after the car ahead of you passes a point on the road, count to three."

Three seconds? I've heard of the two-second rule, which has an entry on Wikipedia:

http://en.wikipedia.org/wiki/Two-second_rule

When did the extra second get added?

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Two seconds is what I learned years ago. (My guess is that I am older than most participants on HN.) In actual practice, if I am more than two seconds behind the car in front of me, some car will pull in from another lane to fill the gap. I'm lucky to keep my desired two seconds most of the time.

I just check how many dashes on the highway... the distance from the beginning of one "dash" to the beginning of another is 40 feet... So at 70 mph you need at least 5 dashes.

The two-second rule is definitely the bare minimum... At 70mph that's 210 ft/s, and depending on whether the car in front of you can stop faster, that might not be enough distance.

Very cool - I did not know this measurement, though it seems painfully obvious now that you've pointed it out. I will start using the lane dashes; thanks.

I always wondered if you need less than two seconds if you're behind a vehicle that can't stop very fast (e.g., a large truck). Your superior stopping rate could make up for the decreased reaction time.

This can fail horribly. Consider that the truck can decelerate faster than its solo braking ability by hitting a wall or another car. Chain reaction pile-ups aren't only about reaction time.

So, you can count on not being forced to collide by the driver acting alone, but it takes some extra margin to reduce the threat of rear-ending by a poor braker following you, and to account for obstacle-assisted braking by the driver in front.

wow I never thought about that. Thanks!

When I was in Driver's Ed, I got told to use 4 or more seconds.

Then you must be older than me, as when I learned to drive in 84 it was 3 seconds. I think is below one now.

I sometimes do a 3 second gap just to see how fast people will fill it up.. which is instantly.

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