I dont think we were meant to know. The driver was probably tired and lost control. it is boring seeing shirly act like a drunken tart episode after episode then kevin who hates her sleeps with her after a row. how stupid can you get.
He was just a guy kevin met on the beach first kevin thought he was a pervert then he realised he wasnt, he got dumped their after a stag night. then they got chatting then the guy stole kevins ferry ticket then got on a bus then it crashed then kevin who was following the bus in his car arrived at the crash scene and he died in kevins arms.