yeh it does... the longer you sit in a congestion, the more fuel you use, so the road becomes more expensive... if you are moving then your MPG increases as opposed to idling for 10 mins creeping down the road...

IMO the only fair way would be to abolish road tax, and stick it all on fuel... the more you use the more tax you pay, the more you sit in congested roads, the more fuel you use, the more expensive they become! then you would see a rise in the ammount of people who turn off their cars when stopped and think about the nex car they are going to get!