@AC-dee and AC-dum
(in a purely Lewis Caroll naming convention way)
To the first AC: I (like to) believe my driving style is both safe and skilled - I keep a two-second gap between my car and the car in front - which means I often have to brake to avoid ramming into people who believe that a 2-second gap makes a perfect space for them to change lanes into - and I monitor both general traffic flow and the cars several spots ahead of my own; generally in the event of a slowdown, I'll be braking/preparing to brake several seconds ahead of the car in front of me.
What I was referring to was the joy of trundling through an average speed camera zone - for instance, the 12-mile stretch of roadworks which used to sit on the M1 at Nottingham. Not only do you have to monitor your own speed, but the traffic bunches up, so you have to deal with people behind you crawling up your boot in a mission to stay at 51mph and people in front braking down to 40mph whenever a SPEX camera appears.
In other words: not fun.
@AC2: that's a fair point about coaches and trucks; I was actually thinking of more "individual" vehicles - white vans/medium sized trucks (e.g. UPS delivery), sales people, long distance commuters, etc.
For instance, I know someone who does a lot of travelling for his work, both inside the UK and abroad (USA, Europe). On a given week, it's not uncommon for him to clock up 1000+ miles bouncing between airports, customers and the head office. With an average speed of 65mph (70mph on the motorways, minus rest breaks and town driving), that works out at around 15 hours of driving per week (3 hours per day); if this dropped to 55mph (e.g. 60mph on the motorways), then he'd be stuck on the road for another 3 hours or so per week, or another 40 minutes per day.
So: there's a personal cost: he'd be more fatigued at the end of each day - and he may have to give up more of his personal time to make up some of the difference. There's an efficiency cost: he can't attend as many meetings in a given day and his work-performance may well be impacted by the increased fatigue. There's a financial cost to the company: between his wages and employee overheads, those 3 hours could cost them around £150 with no benefits to either the company or the employee.
But hey: he'll save fuel, right? Let's say his Volvo TD goes from 40mpg to 45mpg when it's trundling around at 60mph. How much will he save per week? About 2.5 gallons, which is around £17 (based on £1.36 per gallon of diesel). Even if we go up to 50mpg, he's still only saving around £31, at a cost to his health and social life, a higher risk of a fatigue-related accidents and a significant financial cost for his company. Does that really balance out?
Admittedly, he's a relatively extreme case, but he's far from the only person in the UK who does long-distance driving on a regular basis.
It's also worth bearing in mind that this analysis doesn't take the environmental benefits into account: reduced CO2/pollutants from the exhaust and less wear on the tyres (i.e. less latex dust in the air - http://www.straightdope.com/columns/read/2661/when-the-rubber-meets-the-road-where-does-it-go). Quantifying that is far trickier, and there could be long term negative tertiary impacts. For instance, higher costs in the UK could result in work being outsourced to countries with weaker anti-pollution rules...
All told, I'm skeptical about the benefits of this sort of standalone proposal; by itself, it achieves little and potentially costs a great deal. For this sort of idea to have merit, it'd have to be part of an overarching system which reduces the overall need for individual travel and delivery of goods...