I did this on stage 1, wave 1. Place a marker on either the left or right hand side just above the crosswalk (press 'A' on a tile to place a marker) .
Now the tricky part is getting the timing right, you need to detonate the green marker when the car is about halfway across the crosswalk. I counted the seconds the car took between moves (roughly 3 seconds) to get my timing right. If you fail on wave 1, just keep trying the same method until you time it correctly.