If you're like me and you get a bit tired of trying to drive cleanly race after race, then like me you'll want a quick easy way to get this done
to load up the menu, right bumper
over to 'Creative Hub' and select 'Blueprint Events'
From here you're going to want to find a race event called "1 MILLION RUN" by PRLCW044. You can find it in the 'All Time Greats' section. After this select any A Class car you like and start the event.
The blueprint is 5 lap run of the M68 motorway and is designed to slow down AI drivatars with extra turns. That way you can just take the first corner and then floor it straight on the open road. I picked a stock A Class Mercedes, put on my favourite radio station and the achievement popped in 5mins of racing. Ended up taking one and a half laps. Soon as it pops you can quit the event and move on