At present, there is a definite need to cull the human species, what better way than increasing the Darwinian factor? Studies have shown, through extensive testing of laboratory humans, providing this species with additional rope to hang themselves results in an increase in accidental hangings. It is thus reasonable to assume an increase in the imposed speed limits will encourage incidents of accidental steering wheel impalements and vehicle ejections (a colleague suggested including an ejection seat for those humans who frequently fail to put on their seat belts, but a separate study indicated an ejection seat merely expedites the inevitable and is thus an unnecessary expense).
As the expense for increasing the speed limit is marginal, it is a reasonable step to culling. However, an increase of 10mph (the discussion being UK, wouldn't it be kph?) is insufficient to substantially decrease the human population, particularly considering their breeding habits. An equally effective approach would be neutering (a cost effective alternative may be to legalize oversized tires on large trucks. Four studies all determined a correlation between small male reproductive organs and large trucks, while one study found similar results with that of oversized tires. All five studies indicated substantially lower testosterone levels in the males, but an increase in testosterone - and accompanying body hair - in the females. It has been suggested this may not necessarily be the result of owning or driving such vehicles, but instead merely a compensatory consequence).
Further decreasing the size of European vehicles (whilst reducing their manueverability at higher rates of speed) is already in effect, but additional steps will need to be instituted if we're to have any hope of removing dumbasses from the reproductive cycle.