How do you know if you’re going too fast when driving?

Picking the right speed is an important part of driving. There are three concepts:

1. Excessive speed: this is any speed above the speed limit
2. Inappropriate speed: this is any speed below the speed limit, but above or below what is safe
3. Appropriate speed: this is any speed which is safe for the conditions and doesn’t hold up or obstruct other road users. This is also called ‘reasonable speed’.

Therefore, appropriate speed is an ever-moving band. We could look at a hypothetical example of a motorway with light traffic, but it’s raining moderately and visibility isn’t good:

• Excessive speed: 71mph or more (in a car or on a motorbike) – this is above the speed limit
• Inappropriate speed: 0-40mph and 55-70mph – any speed above 55mph and you can’t see a good six seconds ahead of you, while any speed below 40mph is causing holdups and is unreasonably slow for the conditions; other drivers would not expect you to be doing less than 40mph in these conditions
• Appropriate speed: 41-54mph – this is a good window of speed where stopping distances and visibility have been taken into account.

Obviously, that would just be a snapshot of one particular place on that motorway. Inappropriate speed might drop to 45mph at a point where there’s a lot of traffic joining from a slip road, or it might go to 60mph if the sun comes out, but the road is still wet.

It’s the driver’s job to judge this. In fact, we do it constantly without thinking. Every time there is a sharp corner, the limit of inappropriate speed drops and, as the road straightens out, it rises again until there is no upper limit for inappropriate speed, only excessive speed because it’s safe to travel at up to the speed limit.

Therefore, we can think of appropriate speed as a narrow band that moves constantly with the characteristics of the road surface, visibility, traffic volume, curves and gradients, pedestrian activity and more. While we’ve used 15mph as the band in the example above, that is simply an example. On some roads, the band of appropriate speed will be less and on other roads, it’s more. Different vehicles have different appropriate speeds in different scenarios, too.

This band of appropriate speed also contributes to how smoothly traffic flows. It is a band of relatively safe speed which the majority of drivers feel comfortable driving at. When all drivers drive at approximately the same speed, we get the optimum traffic flow.

The majority of injury and fatal accidents happen when drivers are doing inappropriate or excessive speed because mistakes are magnified. Giving drivers the skills to adjust their driving to an appropriate speed reduces the risks of crashes.

Picking the appropriate speed is not a defined science. In fact, it can be defended in common law using the man on the Clapham omnibus argument.