Update: (Again) Some people have said they can only unlock it with the Agency Car and some people have said they have gotten it without the Agency Car. I think there might be a factor on what level your driving skill is. I unlocked this at skill level 1 where a few other people have been unlocking it at skill level 4.
To spawn the Agency Car hit UP
on the D-PAD. I did this on the highway at the very start of the game located at the supply point: Westport Car Park. Go left leaving the car park and don't hit anything and the achievement is all yours. The road has a few bends in it but you shouldn't have any problems. Just be careful the farther you go down the road the more enemies there are and they will start shooting you! Have fun!
Edit: To get the Agency Vehicle you need to get your Driving Skill to Level 1. There is an achievement for unlocking the Agency Car.