Some of the earlier posts said they had a tag number. Who is this tag registered to? If it's registered to Abdul, then why search granny's car?
Colonel Craig Roberts (former SWAT commander, I believe) was on the Alex Jones show the other night, and he suggested the widespread use of unmarked police vehicles (intelligently placed) to block off traffic where the shootings occur.
Of course, they would have to be positioned beforehand, which would be no easy task given the geographic area to cover. A starting point would be those locations which many escape routes as the shooter appears to prefer these locations. Volunteers could help out the local police here.
I don't think that random or universal car searches will help catch the terrorists, since terrorists are used to getting around these tactics in places like Israel.