Speed limit rise: do half of motorists break the speed limit?

Proposals to raise the speed limit on motorways from 70 to 80 mph have caused a stir in this morning's media, with the story adorning the front pages of the Independent, Times and Telegraph among others.

The policy certainly evinced strong opinions among the panellists and audience on last night's Question Time. But is it grounded in firm evidence?

Transport Secretary Philip Hammond told The Times: "I take the view that we operate in a democracy of policing by consent.

"If 50 per cent of the population are routinely breaking the law it's actually the law that needs looking at."

So where does this particular statistic come from?

The Department for Transport collects data on the speeds of various vehicle types using automatic traffic counters, which are used in "free flow" conditions (without traffic congestion, roundabouts, or indeed, traffic enforcement cameras).

Using this information, we can see that 49 per cent of cars were caught breaking the 70mph speed limit on motorways in 2010.

This was higher than the proportion of cars found to be speeding on other types of roads, with fewer than one in 10 cars breaching the speed limit on single carriageways, although the proportion breaking the speed limit has fallen on all road types over the past decade.

It is also worth noting that 14 per cent of cars were already clocked above the 80mph limit proposed by Mr Hammond.

However the average speed of cars on motorways remained below the current speed limit of 70mph.