Road Space Needed For A Vehicle

The space required on a roadway for a moving vehicle that is must be able to come to a safe, controlled complete stop is now easy to tot up.

It will be the length of the vehicle, say 12 feet, to which must be added the distance traveled while the driver reacts, to which must be added the distance traveled while the brakes are applied to bring the vehicle to a standstill.

With the figures we have developed, we merely add the 12 feet to the safe stopping distance to get 311.2 feet.

That’s assuming that we are traveling at 88 fps and our Driver Reaction Time is 1.2 seconds, and have a braking acceleration of 20 fps/sec.

We can now visualize our stretch of roadway, (“Vroom; whoosh; putt-putt, vrooom”) with a series of cars passing us.

We now know the distance we will need between the front of one vehicle and the front of the next vehicle. It is 311.2 feet (given our assumptions about speed, acceleration and driver reaction time).

If the vehicles are traveling at 88 fps, then we will see a vehicle about every three and a half seconds. (311 divided by 88 is 3.53).

Thus, for any given speed, we know how many vehicles per second will pass us by, and that equates to knowing “The maximum number of drivers per second that can be got home safely.”

Of course, as the speed changes, so will the safe stopping distance, and hence, so too will the number of vehicles per second that pass us by as we stand at the side of the road.

We’d like to know how to maximize that number, so we want to know the speed for which that number is a maximum.

All we need to do is to calculate the number for a set of different speeds, then pick the speed which lies alongside the maximum number.

To give you an idea of where we are going, I’ve pasted in a table of results. I’ll show you how to calculate it over the next few pages, but anticipate that fourteen miles an hour is the best getting-me-home speed.

That’s rather amusing, isn’t it, for a 60 mph highway?

Doing The Calculations

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