If a dealer told me that I could only drive a car for 15 mins, I would say “Thank you. 15 minutes should be long enough to tell me if the car is a definite ‘no’. If I think it’s a possible, I will obviously need to go to another dealer who allows longer test drives. If I then decide that I want one, I will naturally order it there”. I somehow suspect that the 15 mins offer may immediately increase! I’m afraid that, from my experience, the majority of car salespersons deserve the reputation that they have and good ones are the exception rather than the rule. This also applies to product knowledge. I research cars thoroughly before I even go to the dealers and almost always find that I know more about the specification of cars than those who are paid to sell them!