Obviously for each road you are wanting to get the highest value of damage by crashing into cars, vans, buses, and overhead road signs. Each of these has a different value with the smaller cars having the smallest value and the value getting bigger depending on the size of the vehicle.
Buses are the key to getting the high value as for each bus you hit you'll get a score multiplyer. You can do most showtime road rules with about 3 or 4 bus hits as long as you are plowing through the normal traffic.
One thing to watch out for when going for the buses is that if a bus appears on screen try not to let it go off screen by turning the camera as this can sometimes despawn the bus or other traffic for that matter.
Try not to bounce up hill as some hills need a good amount of momentum to get up and whilst your doing this you'll probably be bouncing over the traffic, so your boost bar will quickly go down.
Take it slowly and don't keep rattling the ground break button as you can get a lot of momentum from cars crashing into you, and if you are bouncing quite high your going to be missing most of the traffic anyway.
Try to crash into on coming traffic so that it keeps coming at you. You don't want to be going with the flow of traffic as you'll end up chasing it up the street sometimes.
I found enclosed tunnels were very easy to rack up the value as you generally rebounded off the ceiling of the tunnel, keeping you right on top of the traffic. The tunnel can be your normal tunnels or where the rail road runs above the road.
For ever yard you travel during the crash you will also get $100 so make sure you keep moving.
Thanks for the correction!