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The Two Second Rule

Rear end collisions are one of the major categories of automobile collisions.  They appear to account for around 10% of the total number of collisions in provinces such as Nova Scotia, and up to 30% or more of the total collisions in North America generally.  The higher number almost certainly applies to British Columbia, particularly the BC lower mainland.

Rear end collisions have come under a lot of scrutiny over the years.  The general assumptions are that, for the most part, they are preventable, and except in very unusual circumstances; the driver of the striking vehicle is at fault.

Breaking Time

Some exceptions to attributing fault to the driver of the striking vehicle may be an erratic, unsafe lane change by the vehicle in front or the vehicle in front reversing on the roadway.  The key consideration is whether the driver of the striking vehicle had sufficient time to react and apply brakes, and to safely stop.

Brake reaction time varies significantly among drivers.  Some people can brake within one second of detecting danger, while others may take up to 3.5 seconds.  Brake reaction time further varies depending on whether a driver, for instance, is emotionally unstable, sleep-deprived or medicated.  Weather, road conditions, driver distractions and prominence of the threat are additional variables.

Traditionally, 90% or so of drivers can detect a threat and apply brakes within 2.5 seconds. A standard assumption is slightly less than two seconds.

Speed and Distance

At 50 KMH (30 MPH) a vehicle is moving at about 45 feet per second, or about 115 feet in 2.5 seconds.  This means that a motorist in a stream of traffic moving at 50 KMH, should maintain a distance behind the vehicle in front, of well over a hundred feet, or figuratively, about seven or eight car lengths (a “car length” being notionally, about 15 feet).  At highway speeds, this distance should double.

Even on casual observation, it is clear that we do not generally maintain safe driving distances.  We just sail along, hoping for the best.

How to use two second rule

The Rule to Avoid Rear End Collisions

Under ideal conditions, and at the relatively moderate speeds, something called the “two second rule” is a useful guideline for a driver to maintain a minimal distance between vehicles.  The concept is:

  • Pick a large object – tree, sign, lamp-post.
  • As the vehicle in front passes the object, count down two seconds.
  • If the vehicle passes the object in less than two seconds, you are too close to the vehicle in front.
  • Under bad weather conditions, double the countdown to four seconds.
  • On a highway four seconds is a minimum.

So that the two second countdown does not in itself become a source of distraction, do not refer to electronic devices.  Count down verbally, “one thousand one, one thousand two” etc.

In Ireland, a government sponsored website encourages the countdown in terms of “Only a fool breaks the two second rule”, which takes about two seconds to recite.  We do not like to think of ourselves as fools, but with over 30% of crashes being easily avoidable, the statistics are an embarrassing indictment.

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