Speed kills: the difference between 30 and 40mph

It’s 30 miles per hour for a reason

That strapline came from what has to be one of the most impactful advertising campaigns in recent years, certainly in terms of road safety.

The voice of a child, a young girl, in the aftermath of a road accident; narrating the cold hard facts that probably gained a permanent place in the mind of anyone who watched it: “If you hit me at 40 miles an hour, there’s around an 80% chance I’ll die. Hit me at 30 and there’s around an 80% chance I’ll live.”

As a road safety awareness tool, it was extremely powerful: a brutally blunt and harsh warning to any motorist who has ever driven through a 30mph zone at the top end of the 30s and nudging 40mph.

Learning to drive doesn’t stop at ‘Pass’

To many drivers, 40mph feels slow enough, while 30mph is closer to a crawl. But just a few miles per hour really can be the difference between life and death. This is one of the reasons black box insurers (like ingenie) reward policy holders who observe speed limits, while persistent speeding is likely to result in premiums increasing.

Perhaps the only way to change attitude to speed is this continued driving education, to combat the bad habits most drivers inevitably develop – before they cause a tragedy.

The protestation of ‘only doing 38mph’ in a 30mph zone is a weak one, and deserves to fall on deaf ears. It’s a common misdemeanour, but those drivers who are caught out by the cameras and subsequently attend a Speed Awareness Course – offered nationwide in the UK as an alternative to a fine or penalty points on a licence – are soon awakened to the consequences that ‘just a few mph over’ can have.

More speed = less time to react

The faster someone is driving, the less time they have to stop. The Think! road safety website cites the fact that the risk of death is approximately four times higher when a pedestrian is hit at 40mph than at 30mph, and fatal accidents are four times more likely on rural roads than urban roads.

This suggests that motorists are more inclined to speed on smaller, country roads that are not policed and monitored by cameras.

When travelling at higher speeds, drivers have less time to spot unexpected dangers and react to them. In this scenario, the stopping distance becomes the critical element: the total distance you travel before you hit the brakes, plus the distance you travel while the brakes slow you down.

Thinking distance + braking distance = stopping distance

Stopping distances vary according to weather conditions – essentially they are doubled if it’s wet, and may be as much as ten times greater if the road is covered in ice. In normal, dry conditions, the typical stopping distance according to the Highway Code is 12 metres (six metres thinking time, six metres braking distance).

That increases to 23 metres at 30mph, and 36 metres at 40mph. That’s a difference of approximately three car lengths, which can be significant for obvious reasons.

It’s a limit, not a target

Clearly, staying 30mph and under on roads with that speed limit is vital. The RoSPA site has ten tips for staying within that limit, including slowing down when entering villages,, driving in no higher gear than third when in a 30mph zone – and remembering that speed limits are a maximum, not a target.