this post was submitted on 29 Aug 2023
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For car distance I've seen 2-3 seconds start to be recommended, since people are not good at judging distance. So counting how long it takes to reach the same fixed point as the car ahead.
Car length per 10mph is much easier in my opinion. 2-3 seconds isn't based on any speed so by itself is quite useless. Your car's stopping distance changes with speed. It won't always be 2-3 seconds. Let alone counting reaction time at higher speeds.
Seconds is based on speed though. It's literally how speed is defined. Distance over time. At 60mph you travel 88 feet per second. So if you count out the seconds between when the car ahead passes say a road sign and you pass the same road sign and it's two seconds, there's roughly 176ft between your two vehicles. If you're going 25 mph, then two seconds equates to 73ft of space.
Which really gives you more space than car lengths per 10mph, so it's safer. 10mph=~14fps. The average length of a car is 14ft. So you end up giving double the space if you count out 2 seconds.
I cannot not think 14 frames per second when I see that.
The driver can only perceive 14fps
It is in fact, seconds. Not only is your own reaction time fixed, distance is speed × time so the distance you follow is farther, the faster your speed if you count out the same amount of time.
It works for all speeds, plus how do you know how far ahead 6 cars bumper to bumper is when you when going at highway speeds? The lines aren't always a good tell.
It's better to pick a bridge, a sign, an intersection or anything else interesting that the car in front passes, count 0-[x], 1-[x], 2-[x], 3... and so on where [x] is your favourite 4-syllable state or word. Count to 5 in a transport truck or in bad weather, count to 8 if driving a transport truck in bad weather.