What happens when you put multiple tortoises on the road?
Here's a question: what happens when *everyone* drives slower?
On one hand, everyone could improve their MPG - and reduce wear/tear on their vehicle - the engine isn't worked as hard and tyre wear should be reduced. Lower speeds may also reduce the frequency/severity of accidents, too, as people will have more time to react.
On the other hand: cars will be spending more time on the road - they'll physically occupy a given stretch of road for longer.. Roads have a fixed capacity, so this is likely to lead to increased congestion and traffic jams, especially during peak traveling hours. Which in turn could increase pollution and the risk of vehicle failure (e.g. cars sitting stationary and overheating).
There's also a number of other potential side effects - there's a small but potentially critical increase in both personal and commercial costs; the individual spends longer driving, which increases fatigue and the risk of road-rage and aside from the delay to goods delivery (esp. since truck drivers have to abide by the clock), people travelling for work will have to schedule more "dead time" inbetween activities, reducing their effectiveness - and the company may end up paying for this directly if it's an organisation which treats business travel as working hours/TOIL.
All told, has anyone ever done a study of the side-effects of changing speed limits? I know Top Gear recently did a smug "toldyouso" piece on the removal of speed cameras, but they're not exactly the least biased or most scientific observer available!
On a vague tangent: one place I'd love to see the stats for would be the A14 around Cambridge. They originally had yellow-box photographic cameras scattered along the road's length; these were then disabled for a few months while they installed a set of average speed cameras.
The road was PITA when the photo-cameras were in place, as many drivers stuck to an imaginary "60mph" speed limit until they were clear of the stretch, causing vast queues to form behind them. Things seemed to drastically improve when the cameras were disabled, and then went instantly downhill when the new cameras were activated - with the added bonus that driver stress levels shot through the roof due to the need to keep glancing down at the speedo...