Just writing my own solutiuon here as I still struggled to figure out how this actually works and it took me a few times.UPDATE: Helpful update from Cheese Touch is that the road rules are not available as soon as you start the game so if you can't do them yet you'll need to complete a couple of events at the beginning.
East Crawford Drive is the main North/South Road you start on at the beginning of the game next to the Junk Yard at the North East of the map.
Basically face south (whle next to the junk yard) and tap
on the D-Pad 3 times until the street name turns red and it says crash Offline score or something. Being on the 'Online' score meant I didn't get the achievment first time annoyingly.
Then accelerate down the road a small while to build up some speed then double tap both bumpers
a couple times then your car will automatically jerk right and you'll start to fly in slow motion down the road.
You can control the direction of your car with the left stick and also bounce when you hit something/the road by pressing/tapping
. Just keep bouncing and hitting vehicles/things while pushing up on the left stick. It doesn't matter which road you end up on you just need to have started on East Crawford Drive.
As other solution explains hitting buses gives a massive multiplier. I hit a few buses and a ton of cars maybe 30 or so cars and ended up with a score of 3,500,000. I think you have to beat 1,500,000 to beat the road rule.
Hopefully this was helpful. Just thought I'd write this in case anyone was struggling in the same way I was.
No disrespect to other guide as I used it to help me once I had started the challenge.