30/09/2011 08:02 BST | Updated 30/11/2011 05:12 GMT

Speed Limits - Any Alterations Should be Based on Facts, not Politics

So 80 is soon to be the new 70 – or at least that is what Philip Hammond has in mind.

Given that the existing speed limit on motorways was introduced in 1965 a review makes sense. The world moves on and circumstances alter.

But as with so many things in life, there are two sides to the speed limit debate.

It is true that drivers’ journeys are likely to be made quicker if they can go faster (let’s leave aside the problem of congestion, until a bit later) and though drivers might only save a minute here and a minute there on individual journeys, when you tot up all the time savings and translate them in to monetary terms you are probably looking at benefits to the economy worth many hundreds of millions of pounds.

The down side is that more people will die on the roads, something admitted by Mr Hammond. There is also the likelihood that there will be an increase in carbon emissions – cars travelling at 80mph consume around 20% more fuel than at 70mph and hence will pump out 20% more CO2. This said, drivers are already paying a hefty carbon price in terms of fuel duty. If duty is regarded by HM Treasury purely as an environmental tax – which until recently it was – and not a general revenue raiser, then it would be set at about half its current level.

What society has to decide is where its priorities lie. Are we prepared to accept more casualties and more pollution as a product of a change that ministers believe will yield significant economic benefits? Clearly, if you are a road safety campaigner your answer will be no.

It is worth looking at the existing situation on our motorways. The Department for Transport’s own figures show that half of all car traffic already travels above 70mph – and 15% of it above 80mph - and while the police are well within their rights to stop someone for doing 71mph, they tend not to intervene until a driver goes above 79mph at which point he or she has exceeded the 10% + 2mph leeway exercised on a discretionary basis in line with Association of Chief Police Officers’ advice. One unanswered question is whether a new 80mph limit will be absolute or some tolerance will again be factored in for those driving somewhat faster.

Of course all this assumes motorists will actually be able to drive faster. It is one thing to offer up a theoretical 80mph limit but on many stretches of motorway at many times of the day traffic jams mean 50, let alone 60 or 70 or 80mph, is a speed far beyond reach.

Ironically in congested conditions a lower speed limit might actually speed up journey times. Under the Highways Agency Managed Motorways scheme variable speed limits are in force on several stretches of motorway. The idea is that a lower limit actually calms and smooths traffic flow, removing the bunching and stop-start motoring that occurs when there is a free-for-all.

The RAC Foundation is open to change. But we do want to see the Department’s evidence on which they are building their case. And any alteration should be based on facts and not politics.