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by sib 3765 days ago
I would say that 1 car length / 10 mph is actually a very short following distance. If an average car is 15 feet in length, at 60 mph, that is only 90 feet. At 60 mph, 90 feet of following distance is only 1.02 seconds. (1 mph == 1.4667 fps)

When I was taught to drive, we were told that 2 seconds of following distance was appropriate.

This just reinforces your point that the typical human driver leaves insufficient following distance.
3 comments
david-given 3764 days ago
Some of the roads in the UK have chevrons painted on them at intervals. The idea is that at a normal speed for that road, if you can see two chevrons between you and the car in front, you're leaving the right distance.

It's a good idea; the distance is always bigger than you think.

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ptaipale 3764 days ago
Not bad, but you need an element from the square of velocity (for kinetic energy/braking distance) there to be safe. 1 second is fairly okay at low speeds, but at 60 mph (~100 km/h, 27 m/s) two seconds is shortish and if you are doing 200 km/h (as on a German Autobahn) then 2 seconds (100 m) is not safe in my opinion.

(Although the penalty line in Germany is just this, half the speed in km/h to get meters, i.e. driving closer than 100 meters if going 200 km/h gives you a ticket for tailgating)

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sib 3764 days ago
Completely agree with you. Should have stated that i was taught in the US, where it would be ... slightly atypical to be going 200km/h ;) At the time that I took drivers education classes, the US national speed limit was still 55mph, about 88km/h. At that speed, 2 seconds is probably a good rule of thumb for something that's pretty easier for a driver to observe.
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ptaipale 3764 days ago
True of course. At low speeds, the braking distance is negligible, and the margin is needed for driver reaction. When exceeding 60-80 km/h, the amount of kinetic energy and correspondingly the braking distance will start to dominate what is necessary for safety margin.
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msellout 3764 days ago
When I'm not in heavy traffic I give even more space. But unfortunately at a certain density of traffic the aggression caused by a safe distance actually makes it unsafe as people cut you off and try to "retaliate" for the imagined insult.
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