San Ignacio is easily reachable by car. The Western Highway is the best road in the country. There are several gas stations along the route so you have no worries there. Once you reach the sister-city of San Ignacio - Santa Elena- be aware that there are two bridges and each is a one way bridge. You will need to make a right turn, heading west, and run downhill to cross the bridge into San Ignacio.
There is s Shell gas station at the intersection nearest the Hawksworth Bridge. The roads in town are quite passable and traffic is light.
There is really no reason to drive. The town is foot scaled so walking is your best bet for getting about and really experiencing the town.
By car, you leave San Ignacio across the Hawksworth Bridge which is the only suspension bridge in Belize. This is also a one-way bridge.