A proposal to introduce 20mph speed limits across all London’s roads has met with amazement from motorists and motoring associations.London’s anti-car Green party and the mayor have started the campaign, on the face of it to help cyclists and pedestrians, but the wholesale introduction of 20mph limits on all London’s roads threatens the mobility of millions of Londoners daily, particularly out of peak hours when the roads are quiet.The Greens want to turn the rules for setting speed limits on their head, forcing councils to seek legal approval before raising the limit on major roads to 30mph or higher.At the moment councils have to apply to reduce the 30mph limit to 20mph, a sensible limit for many of London’s maze of narrow side streets, but totally inappropriate for the major roads that link London’s historic ‘villages’ and also carry traffic through the capital to the outer trunk roads and motorways, which start in the suburbs.If the Greens and the mayor get their way, many councils will inevitably struggle to find time to raise the limit, leaving motorists fuming at a miserable 20mph, particularly at off-peak times when the roads are not traffic-clogged.Another question is who will pay for the enormous task of replacing all the road signs across the capital?Both the RAC and AA support 20mph limits outside schools and in some residential areas, but both fear the wholesale application of 20mph will cause chaos.“We are very worried about the law of unintended consequences,” said the AA’s road safety expert Andrew Howard. “Many motorists will start to use tiny side streets for the most direct route if they are restricted to 20mph on main roads. And you could end up with the situation where the most inappropriate roads become much busier.”Howard says the example of Hull, where 20mph zones are limited to side roads, with main roads kept at 30mph or 40mph is a good one. “If you’re driving out of the ports at Hull, for example, you don’t see the 20mph limit. But if you’re parking at home you do.”The RAC has a similar view: “We are not opposed to 20mph limits in certain areas like outside schools and some home zones. But we risk making motorists look at speed limits in the wrong way.”Lib Dem MP for Manchester John Leech proposed a national 20mph speed limit in a 10-minute rule bill read out in February. But like most such bills it is stuck in the system, and unlikely to go any further.