Privet, comrades. Just found out nature of the most(maybe all) of "random" boosts i'm getting during the races. The thing is you're getting a random boost in a couple of seconds after bike passes the car, but the conditions below must be followed:

1. Bike's direction should be straight, or in a minor cornering. Drifting breaks the boost.
2. Bike should be on a gas
3. Bike should be on the road
4. (debatable) Bike should pass the car somewhat near it.

Mentioned conditions worked for me in 100% cases on WILD THING 2000 bike(haven't checked other bikes yet). So i always was able to get from 238 to 255 mph after passing a single car.

I think this therory nicely explains 260-275 mph boosts, after a doulbe/triple car pass. Please check Anko's WR from 1:03 min https://www.speedrun.com/road_rash_3/run/m757k2wz

Got 2 additions might be helpful:

1. If bike passes the car and jumps up in the air, boost will be given on landing no matter how long bike had been flying. Bike should land on the road.
2. If on the moment of the boost bike is on a soil or drifting, you get speed penalty by the amount of "boosted" miles

One more addition: If bike bypasses ongoing direction car, boost comes after 2-3 seconds, if car moves by counter direction from bike's direction boosts comes much faster (around a second). Suitable for Wild thing. I guess boosts triggers when bike passes and then rides away from the car on a certain range. So for other bikes boost trigger time can depend.