It's just not high enough. I find it pretty unfair that I pay the same amount of tax on my familiecar that I use for 10.000 km a year as someone who drives is 100 km per day.
I had a citroen C1 once, because the government canceled road tax on such very efficient, small cars. Sadly his law lasted only a couple of years (and it messes a lot with the market value of these cars!). But is was nice that I only paid insurance as long as I didn't drive it. In Poland there is not even a tax on cars, so it is already only in the fuel. It can be pretty nice there to have multiple cars, a big one when you need it, a small one to use a lot. You could even circulate the big one among family, but as it is now in the Netherlands, nobody want to be the owner of the big car.