There are far too many survivors who would hopefully remember the color/make/presence or absence of other vehicles beside them whe they heard the thunk of the bullet hit.
Although it might add to their sick thrills, there is a very real possibility an adjacent driver, if hit, would swerve into their vehicle.
A malvo-type vehicle rigged as a blind could be part of the scenario, but would be most likely parked, on an overpass, aproach ramp, or frontage road, above road level. Without information on terminal trajectories (where the round went after it hit) it is difficult to do anything but speculate.
The direction of the bullet after the initial impact would tell at least some of where to look for the shooter(s), allowing to tell relative elevation, whether they were shooting toward the aproaching vehicle, side-on, or as it was traveling away.
For example, the exact trajectory of the bullet that killed the lady in the passenger seat after passing through the low front of the driver's door. Hard to see how a rising shot came from a great distance.
I must be missing something. If the shooter was driving down the opposite side of the freeway, all he would have to do is roll down his window and shoot with a handgun. That doesn't sound conspicous to me.
Also, if the shooter is using a handgun from the opposite side of the freeway his closing rate on his target is something like 140 mph. Wpouldn't this contribute to the innacuracy?