Personally I struggled with the other two solutions. This is what Worked for me.
Firstly make sure you are in a club so you have more people to go up against. You need to complete the Train In Vain event before you can join a club. The more people the better.
Go through a speed camera at a reasonably low speed. Your speed will be shown at the bottom of the screen and a few seconds later you should see your speed and a rival’s speed with the option to see the scoreboard (you do not need to view it). If your name moves above theirs, congratulations, you just got your first one.
If your name stays below theirs, make a note of their speed, then go back past the camera making sure that you are just above it. When you and the rival are displayed, you should then move above theirs.
Now go through again a fraction faster. You should get a new rival and you can repeat the process until you get 20.
If for some reason it stops displaying your name and a rival, move to a new camera and start low again.
If you make a mistake on approach to the camera and get slowed down, use the rewind a couple of times and try again. I found that 2 rewinds were better as sometimes using just the one meant the camera didn’t flash at all and was out of action for a while.
If the camera do ant flash when you go through, you will need to move to a new camera.
I hope this helps someone as I found this very frustrating and took ages for anything to register. I checked the achievement tracker each time to check it had registered.
Good luck.