In its latest advertisement (Argus July 18) lobby group Unchain the Brighton Motorist asks your readers why, if 20mph speed limits are safer, in 2013 did the number of casualties nationally increase on 20mph roads whilst they decreased on those with 30 or 40mph speed limits.

Well the answer is very simple. As more and more cities, towns and villages across the country reduce the speed limits on their roads to 20mph there are as a consequence far fewer roads with 30 and 40mph limits. So the shift is to be expected.

A deeper analysis of the 2013 Department for Transport national road safety statistics tells us that on roads with 20mph speed limits there were 3,164 casualties, six of whom died, whilst on those with 30mph there were 111,836 casualties, 538 of whom died.

Statistically then the likelihood of death in a collision increased by 2.5 times on a 30mph limit road compared to a 20mph limit.

So the Unchain suggestion that 20mph speed limits are not safer has no basis in fact as the evidence is clearly that they are.

Road safety is a very serious matter and we welcome debate on how best to keep people safe on the city’s roads but please let’s inform that debate with facts and evidence.

Readers will hopefully be reassured to know that the evidence here in Brighton and Hove is very clear. The first year of the 20mph speed limit changes in the city centre saw a 17% drop in collisions and a 12% drop in casualties compared with an average of the previous three years. Most importantly there were no fatalities.

No wonder more and more residents are asking for slower speeds on the streets where they live or their children play or go to school.

Because unlike Unchain they understand that slower speeds mean safer streets.

Cllr Ian Davey Green Party Councillor for St Peters & North Laine Ward Deputy Leader Lead Member on Transport Brighton & Hove City Council