Disposable overalls: $10

That's why they don't use dye in protecting cash, and no-one ever got in jail by getting dyed.

It works great against the unprepared, not so much against people who plan ahead.

Some cars are 89 feet long, so every 50 meters is every third car. That's a lot of overhead. Theft would have to be huge for that to be anywhere near realistic.

A gantry outside of Barstow that plops a device on top of every third (or fifth) car. Outgoing traffic collects them off again.

It would still be less expensive than an armed Marine, and that solution has been effective before.