Crackdown 2

The road and rooftop races can be spotted from a fair distance away... They're the ones with the glowing pillars of light spouting forth from them... Purple lights signify the road races, and rooftop races can be distinguished by the green light pillars..

Once the race starts, make your way as quickly as possible to the next waypoint marker (again, these are just more of the light pillars)... For rooftop races, you will see the next waypoint as a green pillar and the one after that as an orange pillar, so you'll have an idea where you're going one waypoint in advance - I assume it's the same for the road races, but I've only done the one, and I can't remember offhand.

For rooftop races, probably better to have a decent agility stat...

For road races, pick a decent car like the Supercar or the SUV. Don't worry too much about civillians or other vehicles (especially if you're in the supercar as you'll just launch 'em out the way anyway!) The SUV obvioiusly has the benefit of being able to jump over some obstacles!

Road races are purple flags, rooftop races are green flags on map. Go to one, hold A for it to start. Having a higher level in these skills will help alot, so i sugggest first leveling up to atleast level 3. When u start it just go on/through all checkpoints. When u finish achievement will pop. Driving is easier, especially at night.

I have found that in regards to ROAD RACES, it is generally better to do these at NIGHT, as if you are using the AGENCY SUPERCAR, hitting the freaks will not slow you down, there will be no civilian traffic or pedestrians to worry about so you can focus more on where you need to go. Also this will help increase your driving skill as you plough through the freaks.
Finally, the handbrake is your friend, use it to dictate your entry angle into a bend then power down. hope this helps.