Reducing inner-city speed limits to 20mph does not cut the numbers of people killed or seriously injured, according to a report produced for the Department for Transport.

A trial in Portsmouth has shown that dropping the speed limit from 30 to 20mph had no effect on the number of serious casualties, although it did reduce the amount of minor injuries on the roads.

The report showed that the number of people killed or seriously injured actually rose by 1.5 on average after the limit was lowered. However, the report compilers, Atkins consultants, did say that the numbers involved were so low that they would be subject to variations.

While there was no major effect on the amount of serious accidents, the number of minor accidents in the trial areas fell dramatically. The number of pedestrian casualties fell by 16 percent over the period, better than the national average of 13 percent.

The average speeds in the test location also fell dropping on average over the whole city from 19.8mph to 18.5mph.