San Ignacio is easily reachable by car. The Western Highway is the best road in the country and is fully paved and free of pot-holes. 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, if you are heading west, and run downhill to cross the bridge into San Ignacio. Locals apparently know tourists will miss this turn and are quite aware to signal you that you have made a mistake if you try and go the wrong way.
You leave San Ignacio across the Hawksworth Bridge which is the only suspension bridge in Belize. This is also a one-way bridge to leave town, unless the low lying bridge is closed.
2nd Class buses no air conditioning or "trunk" for suitcases run frequently from Belize City; the trip is usually less then 2 and 1/2 hours, in spite of numerous stops. Buses run to Dangriga on the coast in about 3 hours. In the other direction, buses run to the Guatemalan border quite close by, with connections to Flores and Tikal. For times and schedules, see the Belize Bus Blog