I didn't check all of the replies, so I apologize if someone already mentioned this, but I think that the main culprit with the current URL system is that the server lacks any kind of in-game tutorials and warnings for new players. When a fresh dude comes to his very first URL event, he doesn't know what to do, what to expect. Let alone having any idea on how to prepare for it. Once he finally joined the race, he will see other people using their fancy Super GTs and Sultans, without realizing that these cars were properly modified with dyno and performance upgrades, just to match the room score. This system is way too advanced to be used without any guidelines. One more thing that we're all aware of by now, but I guess I should mention it: The average online has already dropped below 150. I think it's the time where we should start doing things.
As a side note, this class system URL may lead us to a (potential) big rebalance for all of the cars that can not be used for URLs at this particular moment. This should give us even more variety than we have right now. Rougly, only two or (at best) three cars from each class can be successfully used for URLs. Changing the other ones would be a great idea, as it would (first) help older players to regain interest for the server, and (second) breath some life into totally unused cars that people always ignore and call shitty. I know some guys who's already left and will only come back if the muscle update or something similar will be released. So yeah..
Anyways, that's just my two cents. Sorry if there's any grammar mistakes.