Los Angeles County’s transit system is poised to become the first in the country to deploy airport-style security measures to screen its passengers. 

The Los Angeles Times reports that the Los Angeles County Metropolitan Transportation Authority (Metro) is rolling out new portable body scanners that can be deployed in response to terrorist threats and during large crowd events like protests and sporting matches in an effort to thwart potential “mass casualty” attacks. 

The scanners can be used to screen passengers using radio waves from up to 30 feet away and are designed with an integrated split-screen display that produces a black square over the part of a person’s body where a gun or non-metallic explosive device might be located. 

Metro currently operates 93 subway and light-rail stations—with many more on the way—and has plans to utilize the mobile devices as necessary across its system. Officials at Metro explained that areas where passengers might be subject to body scanning will be clearly labeled in each station with signs that read: “Passengers proceeding past this point are subject to Metro security screening and inspection.” Plans call for making “randomized” scans of passengers traveling within these zones. Officials at a press conference announcing the plan explained, however, that passengers seeking to opt out of the possibility of being scanned will not be allowed to ride transit from that station. 

The scanners can process roughly 2,000 passengers per hour, according to Dave Sotero, spokesperson for Metro. The figure is an improvement over previous technologies, The Times reports, but likely to fall short of what would be required to process crowds efficiently during rush hour or large scale events. Recent protests in Downtown Los Angeles, for example, have drawn hundreds of thousands of people at a time and have snarled Metro service even without the scanners in place.