Transportation and Vehicle Safety: traveling speed, braking distances, coefficient of friction

Question
how many meters approximately take to stop if you are traveling at 110km/hr, in a standard 4 cylinder vehicle in dry conditions?

Answer
Hello Scott, and thanks for joining All Experts!

When determining the distance it takes to stop a vehicle, several things come into play.

#1
You must account for the time it takes to visually observe the need to stop, such as someone slamming on their brakes in front of you and decide what you are going to do about what you have seen (perception/decision time). This is usually .75 seconds of time.

#2
You must also account for the time it takes to move what you have observed and decided upon into an action such as braking (reaction time). This is usually about .75 seconds of time as well.

#3
Now you must convert Km/h into meters per second. You do this by dividing your speed by 3.6 in metrics, or multiplying your speed by 1.5 in MPH.

If you are traveling 110 Km/h, you are traveling 30.56 meters per second.

Because it takes 1.5 seconds (average) to begin the braking process, multiply your meters per second by 1.5 and this tells you that we will have traveled at least 45.84 meters before braking can begin.

#4
Now we must go to the manufactures graph to determine what the average braking distances are for the vehicle type, weight, and this assumes that constant ABS braking was used.

We also must factor in the type or tires and amount of tread present.

We also must factor in what's known as the coefficient of friction, which is a measure of contact the tire has with the road. Rain, ice, snow, standing water and depth, and numerous other things will come into play here.

Even things such as asphalt or cement, ambient temperature, and whether the road surfaces are new or old. This is all part of accident reconstruction mechanics.

Given your limited info of a standard 4-cylinder vehicle in average conditions would be just guess work here. However, I would speculate that the index speed for stopping as soon as possible from 110 km/h to zero would be roughly 75 meters.

Now we must add in the perception, decision, reaction time. Manufactures will always underestimate this time. I would add the 45.84 meters we found above. The vehicle manufactures would have probably given something around 21 meters.

If you accept my professional opinion, then you would add 75 meters to 45.84 and this would tell you that it will take about 120.84 meters to come to a full stop, or roughly 30 car lengths.

If you accept the manufactures estimate, then stopping distances would roughly be 96 meters, or roughly 24 car lengths.

I hope I have not confused you in my attempt to explain how this works and how we arrive at the numbers presented. Should you have further questions, please write me back!

Terry