Radiological monitors

Although much of the illegal activity at the San Ysidro port of entry involves drug smuggling, Customs and Border Protection is also on the lookout for anyone bringing radiological weapons -- or illegal material -- into the country. These yellow portals, which all vehicles crossing into the U.S. here must pass through, quickly monitor for any such material, alerting officers if any is detected.

However, there are all kinds of legal medical materials that might trigger the system -- such as residual chemicals from chemotherapy or stress tests. So if a vehicle sets off an alarm, officers will conduct a secondary search to quickly try to determine what triggered it.