To the original poster, apparently GT on the PS2 with its current levels of both physical and visual realism is still only capable of simulating 6 cars at a time, not the number needed to simulate a city full of traffic (as in Driver, GTA, etc.) with the kind of accuracy we have come to expect from the GT name. However I agree it would be a nice thing to have one day, and hope the PS3 makes it possible and they don't use all that extra power simply to make the game look better like they're doing with GT3 and 4.
To those who feel street racing is immature and irresponsible and has no place in a serious simulation, I agree it is a dangerous and illegal thing to do in real life. But isn't one of the fun things about a computer simulation the ability to safely try things out that are too dangerous, expensive, or illegal to do in real life? My girlfriend will never let me learn to fly a single-engine airplane for real, even if I could afford to, however I get many hours of enjoyment from Microsoft Flight Simulator 2004, which does simulate a realistic amount of air traffic, and you can choose to either practice proper real-world flying procedures, or just play around for fun weaving around between buildings, etc. Regardless of what happened on 9/11/01, there is still a large number of people in the world such as myself who are mature enough to know the difference between fantasy and reality, and never try crazy stuff in real life, whether it's with cars or with airplanes. So please don't ruin the fun for us.
Those of you who say Gran Turismo only simulates the world of "professional" racing, that couldn't be farther from the truth. It has always been rooted in the idea of starting out with ordinary "street" cars like Honda Civics which you commonly see people driving in real life, and eventually paying to modify them to make them faster. It has expanded the number of real "race" cars included since then, as well as exotic sports cars like RUF and Cizeta, but it is not strictly a "professional racing simulation." For people who want those, there are already tons of simulations available that focus on NASCAR or Formula 1, etc. But Gran Turismo has been and always will be "The Real Driving Simulator" and hopefully will continue to gradually expand to cover all the fun things you can do with real cars in real life.
As for Need For Speed, it (the first one, which carried the logo of Road & Track Magazine) used to be a relatively serious and realistic simulation, yet it still simulated traffic and cop chases. In fact, on the back of the box it said something like "The cars drive like they should, so you can drive them like you shouldn't." But with most of the newer versions since, it has become more arcadey and silly and missing many of the features and options which I liked about the original, and I haven't even bought the most recent version. The first Midtown Madness also seemed promising, but the graphics are now seriously out of date, and the sequel did not have as good a physics engine. Grand Theft Auto 3 and Vice City are very nice looking and still have a semi-believable physics model, but still not as detailed as Gran Turismo and the car models are all pretty much "fake" cars. I haven't had a chance to try the Burnout series.
Finally, for those who mentioned autocross, that is something I have thought would be a good idea for a long time, and it would appear to have finally been included in GT4 as the "Gymkhana" event, although I haven't heard from anyone who's actually tried it yet. So those of you who are lucky enough to have GT4 already, please let us know what it's like.