UK driving theory test · Topic chapter 4 · 5 min read
Chapter 4: Safety margins
Stopping distances at every speed, the two-second rule, and how rain, ice, snow, fog, wind and heat change the space you need. Study notes for the UK driving theory test, built from The Highway Code.
Why this chapter matters
Safety margins is one of three topics that DVSA named in 2016 as most often answered wrongly in the real test. Most of them rest on two ideas: how far a car travels before it stops, and how much more room you need when the road is wet, icy or hard to see. Learn the stopping distance figures and the weather multipliers and you will answer most of them.
Typical stopping distances
Overall stopping distance is thinking distance (the distance you travel before you react) plus braking distance (the distance the car travels once the brakes are on). The Highway Code's diagram linked from Rule 126 gives these figures for a car in good conditions:
| Speed | Thinking | Braking | Overall stopping distance |
|---|---|---|---|
| 20 mph | 6 m | 6 m | 12 metres (about 3 car lengths) |
| 30 mph | 9 m | 14 m | 23 metres (about 6 car lengths) |
| 40 mph | 12 m | 24 m | 36 metres (about 9 car lengths) |
| 50 mph | 15 m | 38 m | 53 metres (about 13 car lengths) |
| 60 mph | 18 m | 55 m | 73 metres (about 18 car lengths) |
| 70 mph | 21 m | 75 m | 96 metres (about 24 car lengths) |
The diagram takes an average car length as 4 metres and says the figures are a general guide: your attention, the road surface, the weather and the condition of your vehicle all change them.
Memory aid: thinking distance in metres rises by 3 for every 10 mph (6, 9, 12, 15, 18, 21). Braking distance grows much faster: going from 30 mph to 60 mph roughly quadruples it (14 m to 55 m).
The two-second rule
On faster roads and in tunnels, leave at least a two-second gap behind the vehicle in front (Rule 126). Pick a fixed point such as a sign: when the vehicle ahead passes it, you should not reach it until two seconds later.
| Condition | Gap or stopping distance |
|---|---|
| Dry road | At least two seconds |
| Wet road | Gap at least doubled; stopping distances at least double (Rules 126, 227) |
| Icy road | Gap up to ten times greater; stopping distances can be ten times greater (Rules 126, 230) |
| Large vehicle in a tunnel | Four-second gap (Rule 126) |
| Stopped in a tunnel | At least 5 metres behind the vehicle in front (Rule 126) |
Never get closer than your overall stopping distance, and drive at a speed that lets you stop well within the distance you can see to be clear (Rule 126).
Wet weather (Rule 227)
- Tyres have less grip, so stopping distances at least double.
- Keep well back to see and plan ahead.
- If the steering suddenly feels light and unresponsive, water is probably stopping the tyres gripping. Ease off the accelerator and slow down gradually.
- Spray from other vehicles reduces visibility.
Ice and snow (Rules 228 to 231)
- Do not drive unless the journey is essential. If you must go, carry an emergency kit (de-icer, scraper, torch, warm clothes, first aid kit, jump leads, a shovel, a warm drink and food).
- Clear all snow and ice from all windows, lights and number plates before setting off, and remove snow that could fall off.
- Drive slowly in as high a gear as possible, and accelerate and brake very gently.
- Brake on the straight before a bend, then steer smoothly round it.
- If the steering feels unresponsive it may mean ice; on ice, tyres make virtually no noise.
- Overtake gritters with care and do not overtake a snowplough unless your lane has been cleared.
Fog (Rules 226, 234 to 236)
- Before entering fog, check your mirrors and slow down.
- Use headlights when visibility is seriously reduced, which generally means you cannot see beyond 100 metres.
- Fog lights may be used then, but MUST be switched off when visibility improves, because they dazzle and can hide your brake lights.
- Do not hang on to the tail lights of the vehicle ahead; they give a false sense of security.
- Keep your speed low enough to stop well inside the stretch of road you can see.
Wind and heat
Strong crosswinds can push cars, cyclists, motorcyclists and horse riders off course, especially on open roads, by bridges and at gaps in hedges (Rule 232). In very windy weather keep well back from motorcyclists overtaking high-sided vehicles (Rule 233). In hot weather the road may soften, and rain after a dry spell can make it slippery (Rule 237).
Skids
Skids are usually caused by harsh braking, accelerating or steering, or by speed too high for the conditions. Remove the cause and steer into the skid: if the back of the car slides to the right, steer to the right (Rule 119).
FAQ
What is the overall stopping distance at 30 mph?
23 metres: 9 metres thinking and 14 metres braking (Rule 126 diagram).
How much bigger should my gap be on an icy road?
Up to ten times the normal gap, because stopping distances can be ten times greater (Rules 126 and 230).
When must I use headlights in fog?
When visibility is seriously reduced, which the Code generally takes to mean you cannot see for more than 100 metres (Rule 226).
Check yourself
- Give the overall stopping distance at 50 mph and at 70 mph.
- How much should your gap increase on a wet road?
- What does unresponsive steering in heavy rain suggest?
- Why must fog lights be switched off when visibility improves?
Practise this topic on its own: Safety margins drill.
Ready to try a full test? Start UK theory practice test 1.
Contains public sector information licensed under the Open Government Licence v3.0. Based on The Highway Code.