That is part of emergency egress training during Terminal Countdown Demonstration Test TCDT activities.

Under the guidance of Capt. Steve Kelly (left), with Space Gateway Support, STS-96 Mission Specialist Daniel Barry (right) practices driving the small armored personnel carrier that is part of emergency egress training during Terminal Countdown Demonstration Test (TCDT) activities. At the rear of the carrier are Pilot Rick Douglas Husband and Mission Specialists Tamara E. Jernigan (Ph.D.) and Ellen Ochoa (Ph.D.). The tracked vehicle could be used by the crew in the event of an emergency at the pad during which the crew must make a quick exit from the area. The TCDT also provides simulated countdown exercises and opportunities to inspect the mission payloads in the orbiter's payload bay. Other crew members taking part in the TCDT are Commander Kent V. Rominger and Mission Specialists Julie Payette, with the Canadian Space Agency, and Valery Ivanovich Tokarev, with the Russian Space Agency. Mission STS-96, which is scheduled for liftoff on May 20 at 9:32 a.m., is a logistics and resupply mission for the International Space Station, carrying such payloads as a Russian crane, the Strela; a U.S.-built crane; the Spacehab Oceaneering Space System Box (SHOSS), a logistics items carrier; and STARSHINE, a student-led experiment