Bridge Constructor Portal

Once you've reached level 20 make sure you have nothing built, if you do choose the option to clear construction. Once you've cleared everything hit drive and your vehicle will keep going back and forth through the starting portals, once it has passed back and forth 30 times quit the level and achievement will unlock.

Please note: Most achievements in this game require you to get to the next cut scene before it unlocks or you can simply quit to main menu to unlock it immediately.