I've long thought that the driver who pulled over with McVeigh was part of some sting operation. If the cars weren't speeding, there would be no logical reason for him to pull over, unless he planned to kill or disable the trooper.
Since he didn't do that it, it is likely he pulled over to make sure McVeigh didn't get away, and to serve as backup for the trooper.
It is inconceivable that a lone trooper wouldn't notice two vehicles pulling over. The only reason for the trooper to protect the driver of the second vehicle would be because they were "on the same side"...