During some Roaming Races the rubber banding effect is so strong it can become impossible to beat your opponent, even when driving highly specialized hypercars.
The best defense is a good offense. To defeat the Roaming Racers of Fortune Valley you must incapacitate them at the beginning of the race.
Bumping into other drivers doesn't impart a lot of force, but it can be enough to direct them into oncoming traffic, a nearby wall, or off of the asphalt. In most cases, gimping them at the beginning of an instigated race will be enough to put me in an irrevocable lead. The game does not have any "safe driving award" for clean racing so there is no downside to shoving your opponent off of the road.
I feel this is the only way to consistently beat Roaming Races. If they gain the lead it's almost impossible for them to lose it, you have to prevent them from taking the lead in the fist place.