Safety Margins Theory Test Guide
- 1 Safety Margin definition
- 2 Stopping Distance
- 3 Typical Stopping Distances
- 4 The two-second rule
- 5 Thinking distance
- 6 The effect of stopping distance on safety margins
- 7 Weather Conditions
- 8 Skidding
- 9 Roadworks
- 10 Contraflow systems
The safety margins theory test section is about driving in conditions that affect the braking and handling of a vehicle.
It is normal for new or learner drivers to struggle to keep good safety margins, when compared to experienced drivers. This means that learner drivers tend to have greater difficulty maintaining a safe distance between their vehicle and the one ahead of them.
As a learner driver, it’s imperative that you always keep in mind the safety of yourself, any passengers you may be carrying and other road users.
You can drastically reduce your chances of being involved in a road traffic incident by knowing about safety margins and the potential consequences if you don’t abide by them. As a learner driver, you mustn’t get in the habit of coasting, which means driving whilst the car is in neutral or with the clutch pressed down entirely – the result of this being less control over your vehicle.
Safety Margin definition
A Safety margin is defined as the space you must leave between your vehicle and the one in front so that an accident will not occur if it slows down or stops suddenly. Safety margins are also referred to as separation distances. The safety margins dealt with in this section primarily refer to following distances and stopping distances of vehicles. A following distance is the space left between your car and the vehicle ahead. Stopping distances include the combined reaction time and braking distance, once you realise you have to stop.
As a learner driver it is common for other, more experienced, drivers to pressure you into speeding up and narrowing your safety margins. While difficult and a little daunting, it is important that you stand your ground and keep a safe distance behind the vehicle ahead of you. This will give you time to identify and react to hazards early.
The driving theory test often confuses learners when it comes to stopping distances. Some questions will ask for overall stopping distances, while others will as for braking distances. The confusing part is knowing the difference between the two:
Therefore, the overall stopping distance is determined by the time it takes you to become aware of a hazard ahead, plus the time taken to brake as you react to it.
Typical Stopping Distances
Below is a diagram of the typical stopping, thinking and braking distances as given by The Highway Code.
The above figures are based on vehicles travelling
- With good tyres and brakes
- On a dry road
- In good conditions
The two-second rule
To keep a good safety margin in traffic moving at normal speeds, a rule of thumb is to allow at least a two-second gap between you and the vehicle in front of you.
An easy way to do this is by noting a stationary object, like a sign or traffic cone, and waiting for the vehicle in front of you to pass it. Once the vehicle passes it, count the seconds until your vehicle passes it, and adjust your speed accordingly.
In other weather conditions, this distance must be increased:
This is the distance travelled during the time it takes for you to notice a hazard ahead of you. It is sometimes known as the reaction distance and it is related to your reaction time.
To find your approximate thinking distance, simply take your speed in miles per hour (mph) – your thinking distance will be that many feet. For example, when driving at 40mph, your thinking distance will be 40 feet. In other words, after a hazard appears in front of you, you will travel a further 40 feet before you begin braking in response to it.
The effect of stopping distance on safety margins
As we explained above, the safety margin is the space you leave between your own vehicle and the one in front of you to ensure that, if they must stop or slow suddenly, you have space and time to stop your own vehicle safely.
When behind long vehicles and motorcycles, the safety margin needs to be larger. This is because these types of vehicle need more space to stop – they have longer stopping distances.
In addition, driving behind these vehicles means that you have to be aware of strong winds as well as the potential hazards they may cause. Long vehicles have large surface areas and motorcycles are light, which means that these vehicles are particularly vulnerable to being thrown off course in strong winds. Again, this requires you to leave a larger safety margin.
Long vehicles can also block your view of the road ahead of them, so you may not see hazards developing beyond the vehicle in front of you, if you are too close. Similarly, the driver of the long vehicle may be unable to see you, another potential hazard, if you are too close to the vehicle’s rear. To enable you to see and be seen, pull back from the long vehicle and increase your safety margin.
To compensate for bad or adverse weather, you must lengthen your safety margins.
In rain, the safety margin you would apply on a dry day should be doubled between you and the vehicle ahead of you. When conditions are icy, your stopping distance is ten times that of your stopping distance on a clear day, so the separation distance must be ten times as long. Applying this to the two-second rule for a clear day: rain uses a four-second rule; ice uses a twenty-second rule.
Many drivers ignore these guidelines, including the lower speed limits on motorways in adverse weather. But it is important to ensure that you stick to this advice for your safety, as well as that of the other road users around you.
Aquaplaning is when the tyres of a vehicle lift off the road and skate along the surface of any water collecting on the surface, causing the steering to become light. This can come about during, or following, heavy rainfall, leaving lots of water on road surfaces.
If aquaplaning occurs:
- Ease your foot off the accelerator pedal
- Wait for your steering to feel normal again before you brake
You should always test your brakes after driving through deep water, such as a flooded area or fording a river. To dry them out, you may need to press lightly on your brake pedal as you continue on the road.
Hot / bright weather
Hot weather can also be potentially hazardous to drivers. The heat can soften the road surface, reducing the efficiency of your braking and steering. In addition, bright sunlight can dazzle drivers, so they may not see the flashing of your indicators. Arm signals may be used, since they might be easier to see in bright sunlight.
Roads can become quite slippery, making it harder to control your vehicle under freezing conditions.
Always clear ice and snow from your car before a long journey. In particular, your windows, lights, mirrors and number plates should be free of ice and snow before you set off.
Once you have set off:
- Maintain a low speed
- Brake gently, gradually and in plenty of time
- If your windscreen wipers are unable to clear the windscreen, be ready to stop and clear it by hand
The most relevant effect of fog on driving is its tendency to reduce visibility. In foggy conditions, you should:
- Plan for more time in your journey
- Keep your speed down since you won’t be able to see as far ahead
- Increase the separation distance between you and the vehicle ahead of you
- Switch on your dipped headlights – in daylight too
If you have fog lights, you must use them if visibility falls below 100 metres (328 feet). When the fog lifts, you must switch them off.
Strong winds, on open stretches of road in particular, can blow your vehicle off course. Their effect is even larger on:
- High-sided vehicles
- Vehicles towing caravans or trailers
Be aware of this hazard and afford these road users extra room if passing them. They may be blown off course by sudden gusts of wind. Be sure to check your left side as you overtake them.
Skidding occurs when the tyres cannot grip the road as they should, and it can be avoided by the driver. While the condition of the road surface and tyres can make skidding more likely, the skids themselves are caused by how the driver steers, accelerates, brakes and the speed they choose to approach at.
The risk of skidding increases when the road is wet or icy. In very cold weather, black ice can form on roads and be very difficult to notice until you feel its effects on your steering as you pass over it – the steering will become lighter.
In order to reduce the risk of skidding in wet or icy conditions:
- Reduce your speed
- Drive in the highest gear you can effectively use
You may start to skid if you brake too hard in these conditions. If this happens:
- Remove your foot from the brake pedal, then gently press down again
- Steer with the skid’s direction; if the back of the car skids left, steer to the left
Scanning the road ahead can help you to identify slippery conditions before you actually meet them. Look out for road signs and markings, as well as other road users slowing down before reaching certain areas of the road. This approach can give you time to prepare for the hazard and plan your driving, allowing you to:
- Gradually slow down to negotiate a hazard (e.g. a bend)
- Steer without sudden movements
Anti-lock braking systems
Anti-lock braking systems (ABS) prevent skidding which is caused by excessive braking. Sensors tuned to the speed of the wheels sense when they are about to lock, and the system prevents the wheels from locking. This allows you to continue steering while you brake.
When braking suddenly in a vehicle with ABS
- Press down on the brake quickly and firmly
- Keep the brake pedal pressed down until the vehicle has come to a stop
Stopping distance is not reduced by ABS because it simply prevents skidding caused by braking which is excessive for the road conditions. Therefore, ABS is less effective when:
- The road surface is covered with water, as after heavy rain
- You’re driving on a loose surface like gravel
Electronic stability control
Electronic stability control (ESC), or Electronic Stability Program (ESP®) enables the driver to turn the vehicle as expected whilst keeping the vehicle under control, as long as the speed of the vehicle isn’t too fast for the given situation.
Even with ESC, there is a risk of losing control if the driver is going too quickly. It is still the role of the driver to ensure that they can stop in the distance ahead of them.
ESC combines the functionality of ABS and traction control systems, and its purpose is to prevent sideways skidding of the vehicle. Advantages include:
- Its ability to activate when the vehicle is about to skid
- Early detection of the risk of skidding, even before the driver begins braking
- Comparison of the driver’s intended direction with the direction the vehicle actually goes
- Selectively intervening with brakes to stabilise the vehicle
All new cars in the UK are now fitted with ESC.
An example of when road conditions are different from normal is when roadworks are being carried out. When signs warn you of roadworks ahead, you must take extra care for the hazards to come.
When a driver in front of you slows down, you should do the same to maintain your safety margin. Even if no hazard is visible ahead of that vehicle, you cannot assume that the space in front of it is safe. Applying pressure to the driver to speed up by ‘tailgating’ is dangerous, as is overtaking into the space ahead. These safety guidelines are particularly important on motorways when you see signs warning of roadworks:
- If the speed limit is lower than normal, obey it
- Using your mirrors and indicators early, make sure you are in the appropriate lane in plenty of time
- Be considerate of other road users – do not overtake queues of slowed or stationary traffic, or force your way into lanes at the last minute
- Always maintain a safe separation distance from the vehicle ahead of you
When traffic in one or more lanes has a direction which is different from the rest of the carriageway, this is known as a contraflow system. This is another example of when road conditions are different from normal.
Upon entering a contraflow system:
- Slow down in good time
- Get in the appropriate lane, as advised by signs for certain exits or if you’re driving a wider vehicle
- Always maintain a safe separation distance from the vehicle ahead of you