From Halopedia, the Halo wiki
A deployable bridge is a portable bridge that can be transported into place and deployed with minimal effort. They are used by the United Nations Space Command in combat zones to allow passage over gaps by friendly forces, and are often carried by transport craft, such as a D77-TC Pelican.
During the Battle at Szurdok Ridge on August 12, 2552, a Pelican transported a deployable bridge to allow SPARTAN-B312 and Catherine-B320 to cross a chasm, replacing a broken bridge that had previously crossed the gap.
The UNSC also employs a portable vehicle access bridge that is often placed in Spartan training sites. These bridges are constructed with practicality and durability in order to fulfill their role in even the most challenging of environments.
Concept art of a UNSC vehicle access bridge for Bloodline.