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Most of us will have been taught the two-second rule when learning to drive.

The two-second rule is a rule of thumb by which a driver may maintain a safe following distance at any speed. The rule is that a driver should ideally stay at least two seconds behind any vehicle that is directly in front of the driver's vehicle.

But what research is there to lend weight to this rule of thumb?

I always liked to think that giving a 2 second gap between you and the vehicle in front was clearly a sensible idea.

Unfortunately, increasingly on British roads I see people driving just a few meters behind the vehicle in front. At 70 miles per hour, 3m puts you just one tenth of a second behind that vehicle. To put that into context, the Stanford press release Sleep deprivation shown to have as much impact on reaction time as alcohol, talks about reaction times in excess of 2 tenths of a second (200ms) for Navy Fighter Pilots, while the wikipedia mental chronometry page suggests a mean reaction time to a visual stimulus of around 190ms for college-age individuals.

Have there been studies looking at the relationship between following distance and accident rates? Between following distance and accident severity? What about between following distance and the number of accidents that would have been near misses if the following distance had been greater?

I have had a look, but it's not my field so don't have much of a clue as to where to start looking for such data or which journals would publish such studies.

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    I would think that there are a lot of studies based on people's reaction times. Also, the automotive industry may have some published reports when talking about the safety features of their cars.
    – JasonR
    Dec 14, 2011 at 17:03
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    I am not sure what you are skeptical of. You seem to believe in the 2 second rule and that it is unsafe to drive too close. I think there is a good question in here but I don't think it meets the skeptics standards which require a single notable claim to be skeptical of. This is almost the start of a good answer to "Is it unsafe to drive to closely?"
    – Chad
    Dec 14, 2011 at 17:08
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    I assume that there is a probability curve describing the chance of an accident (and a smaller one describing the chance of a fatal accident) compared to the size of the gap at a particular speed. It would certain rise steeply as the gap reduces, but I don't expect any discontinuity at 2 seconds. My point is 2s is unlikely to be a magic number, but more a risk-assessment rule of thumb, about where society deems the risk-reward acceptable.
    – Oddthinking ♦
    Dec 14, 2011 at 23:58
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    Think of it this way: if a oncoming car suddenly veered into the other car, causing them to be at a standstill, would you want to be 120ms*60kph away, or 3s*60kph? (Or maybe you would still hit, but at least it would be at <60kph.)
    – Mateen Ulhaq
    Dec 15, 2011 at 3:45
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    I was taught 2 seconds at driving school, but my habit is a minimum of 3 to 5 seconds -- in dark conditions or rainy whether, I tend more towards what I call "the 5 second rule." Prevention is, after all, one of the most effective cures.
    – Randolf Richardson
    Dec 16, 2011 at 3:24

2 Answers 2

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There's a good discussion of the issue of driver reaction time and the factors affecting it (with a couple of references) here: http://www.visualexpert.com/Resources/reactiontime.html

The best estimate is 1.5 seconds for side incursions and perhaps a few tenths of a second faster for straight-ahead obstacles.

Based on that article, I would say that the two second rule should be considered a minimum distance at which to follow and the following distance should be increased as factors dictate. For example, if you're tired, your reaction time will be longer. Allow yourself more distance. If it's dark or rainy or the road is slick, your ability to see an obstacle and stop will be degraded. Allow yourself more distance. If you have a talkative companion in the car distracting you, your cognitive load will be higher and your reaction time will be longer. Allow more distance.

While it probably doesn't have much relation to one's ability as a driver, a fun way to test one's raw reaction time is here: http://www.humanbenchmark.com/tests/reactiontime/

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  • Although Dr Green only cites two of his own papers at the end of his article, and the first now only seems to be available behind a pay wall, he does publish his second paper on his website which has some relevant analysis and this paper cites many more references, so thanks.
    – Mark Booth
    Dec 17, 2011 at 18:07
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    The more you increase the distance from the car in front of you, the more the probability that some car squeezes in!
    – B Faley
    Jan 27, 2012 at 19:02
  • @Meysam: ... and you have to slow down even more! :) So, you're safer.
    – Tom Barron
    Apr 13, 2014 at 11:30
  • @TomBarron: so the most reasonable course of action is to stop. Very safe that way, with infinite following distance! If only everyone would realize this. (sigh) : )
    – user29285
    Oct 8, 2015 at 23:29
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Iowa Driving Simulator studied reaction times in "Driver Reaction Time In Crash Avoidance Research: Validation Of a Driving Simulator Study On a Test Track". This study found that when a sudden threat appears, the timeline for driver reaction was:

  • t=0.00, threat appears
  • t=0.96, driver begins to release accelerator
  • t=1.64, driver begins to steer to avoid the threat
  • t=3.16, driver reaches maximum braking (0.96 + 2.2)

So suppose the driver in front of you suddenly slams on the brakes. If the reaction times found by this study are typical, you will reach maximum braking around 3.16 seconds later. Other things being equal, you will collide with the vehicle in front of you if you are less than 3.16 seconds behind it. The study cites other studies of collision avoidance behavior.

National Highway Traffic Safety Administration looked at the relationship between following distances and accident rates, which you specifically asked about, in "A Front-End Analysis of Rear-End Crashes". This study identified "following too closely" as a principal causal factor in 1 out of 5 rear-end crashes but did not say what their criterion of "too closely" was. If the data provided in this study are reliable, about 162,000 people are injured annually by insufficient following distance, and 600 people are killed.

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    As a police officer of many years experience, it's apparent to me that "following too closely" coupled with "driving too fast for conditions" is the causative factor on an inordinate number of accidents.
    – M. Werner
    Dec 20, 2011 at 19:28
  • Excellent references, but only tangentially related to the problem of the following vehicle scenario. The first paper related to side incursions, which appear to significantly lengthen reaction times, while the second paper concentrates on RE-LVS (Rear-end Leading Vehicle Stationary) scenarios. It's a real shame that the RE-LVm (Lead Vehicle Moving) data set analysed is so small. It's difficult to draw any conclusions from that data other than that driver inattention is significantly more important than following-too-closely.
    – Mark Booth
    Dec 20, 2011 at 19:50
  • @MarkBooth Thanks. What is your source for the claim that drivers take longer to react to side incursions than to sudden stops of lead vehicles? Also, my initial impression of the second study was the same as yours, that it was not relevant to collisions with moving lead vehicles. But then I realized that the LVS data are equally relevant, because "following too closely" was as much a factor in collisions with LVS as with LVM (1 in 5). In other words, those are the vehicles which stopped suddenly and then got plowed into from behind due to "following too closely".
    – MetaEd
    Dec 20, 2011 at 21:24
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    A little addendum: the vehicle in front started braking (even if at maximal breaking power) 3.16s ago... but he won't stop on the spot, giving the vehicle following it more than 3.16s before collision. Not much, but still
    – ADB
    Dec 21, 2011 at 23:30
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    @MετάEd: That's wrong. Two seconds distance means you are fine if you copy everything the driver ahead does with two seconds delay. If your brake lights go on two seconds later than his brake lights, then your car reaches maximum braking power two seconds after the other car does.
    – gnasher729
    Aug 25, 2015 at 22:20

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