And they've devised a cost-effective way of cutting legal limits on all roads in a stroke.
Nia Griffith, Labour MP for Llanelli in Carmarthenshire, wants UK speed signs to be read as kilometres per hour instead of the familiar miles per hour - and 70kph on M-ways equates to 43mph. This would cut the limits by two-thirds without having to replace a single signpost.
The radical call came as MPs discussed climate change last week. "A national speed limit of 50mph would make roads safer," said Griffith. "And I suggest we interpret all signs in kph, so 30mph would be 30kph - around 20mph. And 40mph would mean 40kph, or 25mph. This wouldn't cost a lot.
"We all want our cars and the freedom to use them. But why could we not decide to have a much reduced speed limit? Few people realise how much of an economy they can make by travelling more slowly, but at present they are unable to do that without feeling they are holding up other road users."
Backing the suggestion, Lynne Featherstone, the Lib Dem MP for Hornsey and Wood Green, N London, added: "This is a good idea, and it would be very cheap to implement; the ease with which ordinary people can do things and the cost of the proposals is important."
Meanwhile, MPs have been asked to vote on graduated speeding penalties without knowing what the final upper thresholds would be. The new measures in the Road Safety Bill have been debated in Parliament.
The idea would bring in fixed fines and points for different speeds. It's thought that doing up to 39mph in a 30mph zone would bring two points with up to 45mph attracting three. Drivers travelling at more than 46mph would get six points. But MPs don't know the upper limit over 46mph.
Brian Gregory, of the Association of British Drivers, said: "It's absurd that MPs are voting without having all the facts."