Think out of the box: On top of the SR/DR system --> Drive e.g. 10 clean races and get an exclusive bonus car, 20 races and get a special race suit. Maybe that's an extra push in the right direction.
I'd make it even simpler than that and more direct.
Finish a race without contact = % increase in payout
Finish 3 races without contact = higher % increase in payout
Finish 10 races without contact = even higher % increase in payout
I'd even add a proximity bonus making the payout even higher if you drive a portion of the race close to other vehicles. I'd go so far as to introduce lap bonuses for clean driving as well, bonuses that start from lap 1 and continue until you have an incident and then stop. That'll prevent someone from smashing their way to the front and running clean laps alone to collect the bonus. Heck, make the first lap bonus HUGE. Introduce a first corner bonus as well. Someone who runs a clean race, especially in the first corner and lap, should gather more rewards than someone with a couple of incidents who wins.
If you want to encourage clean racing, increase the rewards for clean racing. Make it so a person that wins get's a lower total payout than someone who drives clean for the entire race and finished 4th. Or someone on a 3 race streak of clean driving earns more prize money for finishing 7th, than someone who wins but has X number of incidents or lowers their SR. Introduce badges and trophies that are exclusive to clean racing and the inability to "
complete" the game (if there is such a thing in GTS) without them. It's a definite must that player lobbies be allowed to use SR and DR as entry barriers.
The
"success" of QM in GT6 made it clear to me that people will venture online because they are motivated by big rewards and quick, pre-organized races. Make clean driving in GTS the best possible way to earn rewards in GTSport and tie it to advancing through the game and you'll get the biggest pool of clean drivers possible. Do like they do now and have a barely working SR system and you'll get what you have now. People taking advantage of the system and making their way through the field while crashing and bashing their way to the front.