DARPA has been using ambulances outfitted with special equipment to keep the nation’s capitol safe from nuclear weapons.

The Pentagon’s research-and-development shop announced on Wednesday the conclusion of its biggest and longest-running test deployment of its Sigma program. The test run had been running for seven months and finished in February.

DARPA installed roughly 73 radiation detectors on emergency vehicles, logging more than 100,000 hours and 150,000 miles around the city. The networked gear runs advanced software that can detect tiny traces of radioactivity, which is able to sort out the benign from the potentially deadly.