If I remember correctly, what you need to do is get the Humvee (Patriot) and approach quickly from the other side of the street. Right before you hit the blip, gently sideswipe one of the racers adjacent to your starting point. The race will then immediately begin and you'll have a head start. Doing the race legitimately in a Patriot is pretty damn easy since you're really fast, can take a lot of damage and have good ground clearance.