Have the vehicles spawn above the starting point.
pawn Код:
PutPlayerInVehicle(playerid, CreateVehicle(model, x, y, z + 5.0, angle, 0, 0, -1, 0), 0);
DisableRemoteVehicleCollisions(playerid, 1);
Where 'model', 'x', 'y', 'z' and 'angle' represent the vehicle model ID, X, Y, Z float and facing angle respectively.
This way, the vehicle falls in to place and is less likely to highly impact on the players below. You could increase the range a little, but an offset too high may result in damage to the vehicle or may cause the vehicle to flip.
Player leaves the vehicle? Respawn/destroy the vehicle and respawn the player. Player leaves the server? Delete/respawn the last vehicle they were in if they were participating in the race.