The same cars are behind, then overtaking, then behind the 2 trucks in the first scene particularly the yellow hatchback.
The yellow hatchback from the first chase scene is behind the truck in the truck fire scene.
In every chase scene, the background changes back and forth from green hillsides to yellow hillsides. That's because the scenes splice together film shot on different freeways and possibly at different times of the year.
During the first crash scene, the camera close-ups on the crashing cars shows that the drivers are stuntmen wearing helmets.
At the beginning the ramp that was used to propel the three cars through the air is visible behind the blue car.
John looks down at the start, sees the 2 trucks and says 'isn't that Robbie's truck?' They chase down the trucks and when Ponch climbs up to the cab, sees the driver and, very surprised says 'Robbie!' He knew whose truck it was.
Sindy's research thesis likely would have been worthless because of a mistake she made at the beginning. She shouldn't have told everyone what their cycles were at the beginning of her project. By doing that it would have unduly influenced their responses, as they would have been more likely to subconsciously tailor their reports to fit the biorhythm chart she had given them. The correct scientific method would have been to have simply told them to keep a daily log of their incidents and then later compared a person's self reported events to their biorhythm chart to see if their was a correlation.