CornishGeek:
I rarely actually do 4.5hrs of driving, although regulary work for 10-11 hours.
If never going over 4.5 hours driving time in a 12 shift then you can take 3 x 15 min breaks as long as the working time between any two of them them does not exceed 6 hours and the breaks have at least a 1 minute gap between start/finish times