fb pixel

Log In


Reset Password

Cop's Corner: Following is based on reaction time

I read an article online that had your name attached about driving distance between cars. Is it two seconds? It seems like everybody would be ticketed because at 55 MPH the distance is 161 feet between cars at two seconds of time. Only blue-haired drivers leave that kind of distance. Is my math wrong? - Casey D., Oregon City

I'm glad, and a little surprised, that my column is being read as far away as Oregon City.

Your math is good, Casey, as you're quite close in your distance estimate. To be precise, it would be 163.46 feet for two seconds of constant-speed travel. But what the Sheriff's Office traffic team is focusing on isn't the distance necessarily, it's the time. The time is what we call perception/reaction time.

There's still the time (and distance) that it would take to skid to a stop from 55 mph, but that's roughly equal for two different vehicles, assuming equal braking efficiency, so we wouldn't be using semi trucks compared to cars, due to semis' having a much lower braking efficiency.

So, getting back to the perception/reaction time, in documented studies a normal unimpaired driver, in daylight, takes 1.5 seconds to perceive a potential threat and then react to it. Some studies have shown an even greater time. That's why the two second following distance is deemed to be safe and thus recommended, with a little room for error built in.

The traffic team deputies have been citing people for following too close when their following time (perception/reaction time) is less than one second, usually less than .75 seconds and as low as .3 seconds at freeway speeds. At less than one second behind another car you'd have to be almost driving 100 percent of the time with one foot halfway to the brake in anticipation of something happening, like having just seen a deer in the road and suspecting there might be another in the immediate area, so you'd be on high alert for a while. However, most people don't drive that way: They are listening to the radio, talking to passengers, talking on cell phones, eating or drinking, tending to children, daydreaming about work or vacation, etc.

So, the proven standard of 1.5 seconds for normal reaction time is the criteria we use. Then we're spotting drivers an extra third of that time before we even consider pulling them over. That way it would be very hard for them to argue that they could have stopped, given normal reaction times, before they would have crashed into the car in front of them. Now it's just a matter of getting drivers' attitudes toward a proper following distance changed.

That's why we have the new equipment - to help drivers become aware of what that distance is.