From Halopedia, the Halo wiki
The Siege Bike is an all-terrain, single-traction tread vehicle designed to target well-fortified enemy strongholds and barricades.[Note 1]
The siege bike is one of several experimental vehicles developed by ONI's REAP-X department, it is equipped with a side-mounted M268 heavy machine gun with spectrum-enhanced visual tracking functionality. The vehicle is built around a heavy-grade, single-traction tread that allows the bike to be useable in all terrains and achieve speeds of up to 145kph (90mph). The bike is only capable of carrying one operator, who rides atop the vehicle.