The bulldozers are machines used is the excavation site of Polar Island. They are used to move heavy rocks or big quantities of dirt.
Bulldozers that are being used by workers attack Twinsen as soon as they see him. They are invincible and very fast, so Twinsen has to avoid them running away.
Twinsen uses one of the bulldozers getting the ignition key from a grobo at the site. He boards it and uses it to move some rocks that are blocking a path to a part of the site that is otherwise inaccesible.