I've tried this a few times. It's not quite satisfying. First off, it requires some low grade distracted driving. I should be paying attention to road, especially the car ahead of me, as well as other cars around me and the traffic signs, but instead I'm counting as I watch things pass by on the side of the road. Because of this divided focus, the method is also prone to inaccuracy.
After having seen enough accidents that were almost certainly made worse because drivers were following too closely and read about a few more, I became curious what the actual safe distances were. If I am driving at a certain speed behind another car driving the same speed, exactly how many metres does a 2 second gap translate to?
The instruction is common enough that I figured I should be able to Google it and find these values for some common speed limits. All I found was the same instruction, whether from government transportation agencies or driving enthusiasts. No values were given, just the same method for computing them, if anything.
There is another method we can use, though: math. In particular, the kind I learned in high school chemistry class, namely dimensional analysis. Using this technique, it's pretty easy to convert from kilometres per hour to metres per second (details below). Multiply by 2 seconds and you get the distance.
In Canada, the speed limits typically run from 40 km/hr to 120 km/hr (typically on the 10s). These speed limits, their equivalents in metres per second, and the resulting safe minimum distances are summarized in the following table (rounded to the nearest 10th).
| km/hr | m/sec | metres behind |
| 40 | 11.1 | 22.2 |
| 50 | 13.9 | 27.8 |
| 60 | 16.7 | 33.3 |
| 70 | 19.4 | 38.9 |
| 80 | 22.2 | 44.4 |
| 90 | 25.0 | 50.0 |
| 100 | 27.8 | 55.6 |
| 110 | 30.6 | 61.1 |
| 120 | 33.3 | 66.7 |
Here's a similar chart for imperial measurements, with miles in places of kilometres and feet instead of metres.
| miles/hr | feet/sec | feet behind |
| 20 | 29.3 | 58.7 |
| 25 | 36.7 | 73.3 |
| 30 | 44.0 | 88.0 |
| 35 | 51.3 | 102.7 |
| 40 | 58.7 | 117.3 |
| 45 | 66.0 | 132.0 |
| 50 | 73.3 | 146.7 |
| 55 | 80.7 | 161.3 |
| 60 | 88.0 | 176.0 |
| 65 | 95.3 | 190.7 |
| 70 | 102.7 | 205.3 |
| 75 | 110.0 | 220.0 |
| 80 | 117.3 | 234.7 |
Having these numbers isn't the perfect solution to knowing how far back to to stay behind other vehicles either. It's not like we can get out and measure while we're driving, and I don't think we're great judges of distance, especially when we're driving. Nevertheless, I think it can still be informative.
For example, suppose we are driving at the lowest speed on the first chart (the typical speed through a school zone in Ontario). The bumper to bumper length of a Honda Accord, a mid-sized sedan, is a little under 5 metres. At that speed, the space between me and the car in front of me should be at least big enough to fit an Accord, and there should be plenty of room to fit another in front and another behind (of course, it may be difficult to manoeuvre those cars into that space at that speed, and if we do, we're well out of the realm of safe distances).
A highway speeds (100 to 120 km / hr), the numbers are more than double that. The gap between cars should be larger than the length of an Olympic sized swimming pool. And yet, it's not uncommon to see cars on the highway driving so close that there isn't room enough for a single car between. In other words, people aren't even keeping the safe distance that is supposed to be used in a school zone.
I was pretty surprised to see these numbers. I generally thought I was keeping safe distances. It's certainly true that most of the time I'm keeping a safer distance than a lot of vehicles I encounter. But it's clear, after doing the calculations, that I'm not keeping as far back as I should be.
There is a third option to consider for calculating safe distances. Don't require drivers to do it themselves. The required technology has been around for a long time. Though I expect it will be an essential component of self-driving cars, it's rarely been applied to cars human-driven cars.
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There are 1000 metres per kilometre, 60 minutes per hour, and 60 seconds per minute. If we are driving X kilometres per hour, that works out to $(X km / hr) \times (1000 m / km) \times (60 min / hr) \times (60 sec / min) = (1000/3600)\times X m / sec = (5/18)\times X m / sec.$
The calculations for feet per second are similar, with 5280 feet per mile in place of 1000 metres per kilometre, yielding
$(X mi / hr) \times (5280 ft / mi) \times (60 min / hr) \times (60 sec / min) = (5280/3600)\times X ft / sec = (22/15)\times X ft / sec.$
