What this topic covers
Margins are the space and time you keep in reserve. Learn stopping distances, the two-second rule, and how rain, ice and fog change everything.
Practice questions
Q1What is the typical overall stopping distance at 30 mph in good, dry conditions?
At 30 mph the typical overall stopping distance is about 23 metres — thinking distance plus braking distance combined.
Q2In wet weather, stopping distances are:
Wet roads cut grip, so allow at least double the dry stopping distance. In ice it can be up to ten times greater.
Q3What is the 'two-second rule' used for?
On a dry road, leave at least a two-second gap to the vehicle in front. Pick a fixed point and check the gap — double it in the wet.
Q4You're driving in fog. What should you do?
Use dipped headlights (and fog lights if visibility is below 100 m). Slow down and leave a bigger gap — full beam reflects off fog and dazzles you.
Q5After driving through a ford or deep water, you should:
Water makes brakes less effective. Test them gently at low speed once clear; lightly applying them helps dry them out.
Q6On an icy road, stopping distances can be up to:
Ice drastically cuts grip — stopping distances can be up to ten times greater than on a dry road, so slow right down and leave huge gaps.
Q7Why should you leave a larger gap behind a lorry in heavy rain?
Spray from large vehicles hides the road ahead, and wet surfaces increase stopping distances, so drop further back than usual.
Q8Driving a long way with the clutch down or in neutral ('coasting') is unwise because you:
Coasting removes engine braking and reduces your control, especially downhill. Keep the car in the right gear.
Q9Your thinking (reaction) distance gets longer when you are:
Tiredness, distraction, alcohol and drugs all slow your reactions, so you travel further before you even start to brake.
Q10In wet weather, the two-second gap to the vehicle ahead should be:
Wet roads roughly double stopping distances, so increase the gap to at least four seconds — and even more in ice or snow.
Q11The typical overall stopping distance at 70 mph in good conditions is about:
At 70 mph the overall stopping distance is roughly 96 metres — about 24 car lengths — so keep well back at speed.
Q12Tyres with very little tread will, in the wet:
Worn tread can't clear water, so grip drops, stopping distances grow and aquaplaning is more likely. Keep tread well above the 1.6 mm minimum.
Q13As your speed rises, most of your overall stopping distance is made up of:
Braking distance grows much faster than thinking distance as speed increases, so at higher speeds it makes up most of the total.
Q14A vehicle overtakes you and pulls in close ahead. You should:
Gently drop back to rebuild the gap you need to stop safely, rather than sitting too close behind them.
Q15Wet manhole covers, painted road markings and tram rails can be:
Smooth wet surfaces offer little grip. Take care braking or turning on them, and give riders extra room.
Ready for the real thing?
Mix this topic with all the others in a full, timed mock test.