How to Test Vehicle Stopping Distance

Before you even think about running any vehicle through a braking distance test, you should consider several factors that will inevitably affect the outcome. It's more than just mashing the brake pedal. Let's take a look at each briefly.

Tires are one of the most important elements when stopping a vehicle quickly and efficiently. When engineers calculate braking distance, they use several variables. One of them is traction coefficient. The higher the traction coefficient is, the shorter the braking distance will be [source: Jones & Childers]. Thus, the braking distance can change greatly based on the type and condition of the vehicle's tires. Tires with little or no tread will be more susceptible to skidding during heavy braking. When the tires skid (a decrease of the traction coefficient), they lose traction and increase the braking distance. Tire compound or makeup can also change braking distance. Some high performance tires offer better adhesion under heavy braking and won't break loose or skid as easily as harder tires.

Tires work hand in hand with the vehicle's braking and suspension systems. If any of these components aren't up to specification, the vehicle's braking distance can change. Worn brake drums, rotors, pads, shoes or leaky brake lines will have an adverse affect on braking. And since weight transfer has so much to do with braking, worn shocks and springs will only add distance during braking. When these components are worn, the weight wants to move to the front of the vehicle when you hit the brakes hard. And we've already illustrated how weight contributes to braking distance.

Finally, road conditions will affect braking distance. Dirt and gravel roads don't offer good traction and increase braking distance. Freshly paved asphalt offers the best adhesion. Consider the weather too. Wet roads increase braking distance. Snowy and icy roads decrease traction even more. Driver ability and whether or not the vehicle is equipped with an anti-lock braking system (ABS) must also be taken into consideration.

These conditions all factor in braking distance. As you'll find out next, the driver plays a key role in all of this too. In the next section, we'll take all of these factors into account and go through a braking test. Let's get to it.

Let's set up a typical braking test considering the factors we discussed in the previous section. For this test, we'll assume the vehicle is in good working order. Let's also assume the road is dry asphalt and in good condition. Mark a beginning point and physically measure the distance from that point to the vehicle each time. Now let's get down to business.

If your vehicle has an anti-lock brake system (ABS), conduct the test with it both on and off. Typically auto manufacturers conduct braking tests at 60 mph (96 kph). We'll do the same. Find a closed area void of traffic and obstructions. Once you get the vehicle up to the test speed, hit your brakes. Without anti-lock brakes, you can easily lock up the front tires and end up skidding, which can easily translate into loss of control and even a spin. Your goal is to hit the brakes as hard as you can until just before the tires start to skid. Keep two hands on the steering wheel and avoid overcorrecting if the rear of the car wants to come around. Conduct this test a few times to get the feel for your braking system.

Next, turn on the ABS and conduct the test again. With the ABS on, the car will feel different when you hit the brake. It's because the ABS pumps the brakes for you to the point where the tires are on the verge of losing traction. Sensors use feedback to dictate how much brake to use to stop the car as quickly as possible while avoiding tire skid. Some antilock systems even have a brake assist that takes over from the driver and applies the most amount of stopping power available [source: Edmunds].

Each driver has different abilities. Some have better reflexes than others; some are more experienced. The best thing you can do is give yourself room between your car and the vehicle in front of you and pay close attention when you drive. A good rule of thumb is a three to four second gap. Add more distance if the road is wet. You can figure this out by counting the time it takes for you and the vehicle ahead of you to pass a certain point on the road.