6.2 Following Distance and Stopping Distance

The test papers return to one idea again and again: the faster you drive, the more distance and time you need to stop. (They ask it directly, as a true-or-false question. The answer is true.) This section turns that idea into numbers you can use.

Two parts of every stop

$$\text{Overall stopping distance} = \text{Thinking distance} + \text{Braking distance}$$

• Thinking distance is the distance travelled between seeing the hazard and pressing the brake. It depends on your speed and your reaction time.
• Braking distance is the distance travelled after the brakes are applied. It depends on speed, the road surface, tyres, brakes, load and slope.

Thinking distance is easy to calculate

Convert km/h to metres per second by dividing by 3.6, then multiply by your reaction time:

$$d_{\text{think}} = \frac{v}{3.6} \times t$$

At 60 km/h, $v/3.6 \approx 16.7$ m/s. With a reaction time of 0.7 s, thinking distance is about 12 m. If you are tired or distracted and take 1.5 s, it becomes 25 m, before the brakes have even started to work.

Braking distance grows with the square of speed

Braking distance is roughly proportional to $v^2$. Double the speed and the braking distance roughly quadruples. Triple it and the braking distance is about nine times as long. That is why a small increase in speed makes a large difference in a collision.

Typical stopping distances

Barbados regulation-test material uses the standard Commonwealth figures for an alert driver in a well-maintained car on a dry road. They were first worked out in miles per hour, so the km/h values below are conversions.

Note how braking overtakes thinking as speed rises. At 80 km/h, the speed allowed on signed highways, an alert driver needs over 50 metres to stop on a dry road.

The two-second gap

A time gap scales with speed automatically:

1. Pick a fixed point ahead, such as a lamp post or a shadow.
2. When the vehicle in front passes it, say "only a fool breaks the two-second rule."
3. If you reach the point before you finish, you are too close. Drop back.

Older Barbados test answers express the gap in vehicle lengths for each step of speed, with more on wet or dusty roads. The two-second method gives the same protection and is easier to use.

What makes stopping distances longer

Being followed too closely

If someone is tailgating you, do not brake sharply to "teach them a lesson." Increase your own gap to the vehicle ahead, so that you can brake gently if something happens. Let the tailgater pass when it is safe.

Worked example

You are driving at 64 km/h on a dry road, two seconds behind a van. The van brakes hard for a dog.

• In two seconds at 64 km/h you cover $\frac{64}{3.6} \times 2 \approx 36$ m, so your gap is about 36 m.
• Your thinking distance is about 12 m, so when you start braking the gap has shrunk to about 24 m. The van has started braking at the same moment you saw it, so it is also slowing.
• Because the van and you brake at similar rates, the two-second gap absorbs your reaction time with room to spare.

Now repeat the example on a wet road with only a one-second gap (about 18 m). Your 12 m thinking distance leaves only about 6 m, and wet braking takes longer than dry braking, so a collision is likely. That is why the gap must grow when conditions get worse.

On long descents

Brakes that are used continuously on a long, steep descent overheat and lose power ("brake fade"). Select a lower gear before the descent so that the engine helps to hold the speed, and use the brakes in firm applications rather than constantly.