Stopping distances might seem like a simple thing to calculate. It seems like you could take a car going a certain speed and press the brake until it stops, measure that distance and then you know the stopping distance for that speed. However that is actually a fallacy. The real world stopping distance of a car involves factors that are much more complex than that.
The main point of a speed limit is safety, of course. The faster a car is going, the less time a driver has to react to avoid a collision. You should remember that stopping distance has to do with a lot more than just the abilities of a vehicle’s brakes. A potential hazard has to be seen by the driver, identified, and the driver has to make a decision about how to react to the situation (I.e. Braking) before the vehicle is stopped. That means as a vehicle moves faster, the potential stopping distance is increased dramatically even for small increases in speed.
We’ll start with 25 miles an hour, which is a common speed limit in urban and residential areas. It may not seem like you’re going that fast, but you are going about twice as fast as an average person can run. Record setting Olympic runners have managed to reach speeds around 25 mph, but that is rare. Low speed zones are established because of the mechanics of the traffic in a particular area. If the speed limit is 25, that means that you are likely to encounter an obstacle in the road (such as a pedestrian) and you need time to react.
Here is a hypothetical situation: A vehicle is travelling 25 miles per hour, and a pedestrian enters the roadway. It takes two seconds for the driver to see the pedestrian, decide to stop the vehicle, and then press the brake. That means the before the driver has time to react, the car has continued moving at 25 mph for 2 whole seconds. The vehicle has moved 55 feet before they even press the brake. If the car has an average stopping distance from 25 to 0 of 30 feet that means that the car will have moved a total of 85 feet down the roadway before it comes to a stop. That’s the length of 8 Toyota Camrys parked end-to-end, and that’s under perfect road conditions.
Because of this human factor, as speeds increase, the stopping distance increases dramatically. At 30mph the stopping distance is much greater—109 feet. At 35 mph it goes up to 136 feet, and you’re not really speeding yet. Switch up the numbers to freeway speeds—60 mph has a stopping distance of around 305 feet. That’s the length of an entire football field to stop. It gives you a new appreciation of why we have speed limits and what they do to keep us safe.