In the Belize City bus terminal, look for a westbound bus; the final destination of BENQUE will be displayed in the front window of the bus, but you could take one with a final destination of BELMOPAN.
The bus companies authorized to operate on the highway in the Western Zone are NTSL, BBOC, D and E, Guerra’s Bus Service and Shaw Bus Service. You could also take a James bus to Belmopan.
Westbound buses depart hourly (sometimes on the half hour) beginning at 5am; last departure at 9 pm.
A Regular bus from Belize City to Belmopan (50 miles) is about 1.5 hours & BZ$5.00.
Coming back, buses to BC depart about every hour.
You can walk from the water taxi terminal to the bus terminal in BC, or take a taxi for BZ$6.
More details here: http://belizebus.wordpress.com/