People will spend half their time concentrating on their speedos instead of looking at the road.
People always poo-poo this argument - "No-one is that bad a driver", or "It just doesn't happen", or "Keep up with the traffic, you'll be alright".
However - I have first-hand experience that speedo watchers DO have accidents, BECAUSE they are watching the speedo and not the road.
About 10 years ago I was working with TRL (Transport Research Lab) as a roving accident investigator on the Thames Valley Traffic patch - incorporating the M4/M40/M25 sections of the motorway network. One bright sunny morning we attended an incident on the M4, just at the M25 junction, eastbound. If you know the area, you'll know there's a slight left curve just as you approach the M25 slip-road.
There was a stationary queue, due to weight of traffic joining the M25, and so the overhead gantries for at least a mile or more were showing a 40mph limit (as they do).
The accident we attended was caused by a driver piling into the back of the stationary traffic at 40 mph.
I know for a fact that it was caused by the driver watching the speedo and not the road and traffic conditions because her very words when describing the accident to me were "But I was only doing 40 miles per hour!"
Driving a car is a hazardous occupation that requires full concentration and a high level of situational observation and awareness at all times. We, as a society, now have such safe vehicles that we have forgotten this very salient fact.
/steps off soapbox