Take the discharge pipe out of your mouth...
Puleeze... If you want to ENCOURAGE economic cars, don´t ban cars that can go over 100mph, cut the taxes on smaller engines. Here in Brasil (with an "S", got a problem?), there was a major tax discount for any car equipped with engines of 1 liter (1000cc) or below. Yes, that´s the size of the engine for a ultra-sport bike. Any North American driving an oil rig, er, SUV, would think that this size of engine are for mopeds only.
It turns out that the rest of the planet can´t afford to be so careless about the environment, and besides, gasoline is WAY MORE EXPENSIVE HERE. Here it costs some 1,3 dollar per litre (some 4 or 5 litres make a gallon, so...). It turns out that it makes a lot of sense to buy a small engine'd car.
It turned out that every car manufacturer was encouraged to extract as much possible of that 1 liter. Now, we have SUPERCHARGED chars on that engine size, built by FORD. It can draws 90hp out of 1 liter. Scale that to a "small block" Mopar 7-liter Hemi, do you get 630hp, supercharged or not? I GUESS NOT.
What about mileage? Most cars that size (1.0L) can do some 16 kilometer per litre, or better. A SUV capable of 5 kilometers per liter is called "economic".
Another thing, "cars must be heavier to be stable and safer at 100mph". Ouuuchhhh. Tell that to Audi TT. Tell that to Mazda MX-5.
Besides that, any 1 liter car, with some tinkering and tweaking (nitro and big-size turbo) can do 100mph. Bikes, by definition, can do 100mph, if they are any larger than 500cc. Will they be banned by 2012? They are greener than a Ford Explorer or Cadillac.
Even a 1972 Volkswagen Beetle (1600cc) with some tinkering, can go 80+ mph, and it barely reaches 70 hp or anything like it.
I guess I have covered all grounds, any single argument to ban cars above 100mph was destroyed. You have to make them so expensive to buy, and anti-economic to keep running, that you will be encouraged to buy something cheaper. It worked around here like a charm. 2-Liter engines here are "Luxury" models. They still have some power-to-weight ratio, and still can reach 80 mph with some effort. People buy larger engines for their ACCELERATION, not top speed, it is illegal to go at 100mph in 99.5% of the planet anyway, the top speed comes as a bonus.
Power limiting (Japan style) = failure. All power limiters can be removed.
Weight limiting = unpratical. No wonder nobody tried that.
Speed limiting = always existed. But it is down to any driver's judgement now.
Engine size scaled tax = excellent. The smaller, the cheaper. No one is preventing you on driving at 100mph, but it will cost you some dosh.