I was thinking about this again this morning. If the practicalities are such that testing everyone every few years is unrealistic, perhaps the assesments should be aimed instead at those at risk. Statistics show that once you've been involved in an accident, you're more likely to be involved in another (regardless of blame, but the correlation gets even stronger if it was your fault), so it's these people who should be targeted first for reassesment.
Similarly with speeding - following on from the thread about Speed Seminars - is a classroom session really the best way to get a message across, or should there be some element where their driving is assesed too (with further penalties/training if it's not up to scratch)?
I've sometimes thought before that if people involved in accidents were given points on their licence (not a fine) it might remove serially dangerous drivers from the road (philisophical question - if you have an RTC does that automatically mean at least one party was DWDCA, because if everyone was paying attention there woulnd't have been a crash?) One of the key laws in aviation is that being in the right does not absolve you of responsibility for avoiding a collision with another aircraft. I guess the difficulty is sorting out "blame" and it would be hard on those people who were innocent parties.
My biggest worry with any of these measures though is that it would simply increase the number of hit-and-run accidents