Btw, which music have you used? The one from the US Gold version on tape or the melodies from the Pony Canyon version on cartridge?
No, I used a midi file which was an adaptation of the arcade music, and rendered it on 9 FM voices.
Perhaps the Arcade sound can be used, as it is made for YM2151 and SegaPCM as VGMrips says. As you know, the YM2151 is in the Yamaha SFG modules. Perhaps a version can be made that supports the original arcade music? 
I'm sure Grauw can tell us more whether this is feasible, as he did similar things for Ghosts 'n Goblins and VGMplay of course.
Indeed, why not ?
Although I must say that I have struggled to write a light music replayer. My current version (PSG/FM) uses up to 11000 cycles per frame (this is for a peak frame, loading and playing all 9 FM notes). If an YM2151 driver can be added to my replayer and give the same performance, it would be great !
Grauw can tell It's only 8 channels by the way. But I don't know how much the SegaPCM is used and important. Emulating that will probably be tricky or at least costly, I guess.
(...) But that buffer size it too big, because it implies that I have to update on ISR between 150 and 250 traffic objects (and check if they are on screen). I already thought about spawing traffic items in a reduced circular buffer, and it might be the solution.
Keeping 150/250 elements seems unpractical, indeed! I would keep about 2x the visible objects (so if you can see 4 cars at the same time, the buffer would be of 8 cars). That will drastically reduce both size and processing time.
If you absolutely want to keep the same cars at the same positions, just store that ("the 37th car is a truck") for spawning the "correct" sprite, but I think that is no necessary; nobody will notice if the fourth car that overtakes him after a crash was a beetle instead of a truck 
