I'm not familiar with 2000AD or Judge abilities, but here are some cliché events to deal with:
- Somebody shoots at the car with a pistol
- Somebody shoots at the car with a bazooka
- Somebody shoots a dart at the tires, to give the car a flat and make it stop. (Best in a city so the ambushers can come thundering around the corner.)
- Plant bomb under car before trip begins
- Minefield or single IED along deserted stretch of road
- "Motorcycle gang" drives by but does not bother car; this is a look-see mission to make sure this IS the car they want.
- Message warning of ambush arrives just as group gets into car and starts moving
- Does the car have an escort of other vehicles? The parade draws attention that the car alone would not.
- The PCs get to drive ahead and check the road for mines &c before the car comes by
- This is all a decoy and the real witness is going somewhere else
- Old wrecked vehicle is site of an ambush - especially if vehicle is a bus or truck that can be reinforced on the inside
- Stop for dinner and somebody - roll randomly - gets food poisoning. Obviously aimed at the witness
- Stop for dinner and somebody - roll randomly - gets food poisoning. The cook really is that bad at his job.
- PCs find a pistol taped under toilet in restaurant (homage to a scene in The Godfather)
- The witness finds a pistol taped under toilet in restaurant. How cooperative is (s)he, really?
- Local petty criminals 'mark' the group and put out word. Now the usual chatter and activity and stuff you want to blend into to hide, is just gone.
- Car breaks down because nobody took it in for 25,000-mile preventative maintenance
- Car is sabotaged to break down in the middle of nowhere. Dehydration in the desert kills the witness just as dead as bullets.
- Car is sabotaged to break down swiftly because the local car theft ring wants to put it through their chop shop.
- Staying at a cheap motel for the night? Oops, it is where the local petty criminals stay, too; poor fieldwork.
- The Cat's Meow is down the block from your hotel, and somebody sniffs out that you have money / supplies / influence.