They're meant to use Batteries till a certain speed - I think its literally like 5-10mph on buses - then a small diesel engine kicks in to help them out. The idea was made mostly because at the time, fully electric was just not fesible. However, what no one could predict was how knackered they'd get. Hybrids were replaced by fully electric or a form of smarter systems where a diesel bus will use regenerative braking. Then most Hybrids now are being converted back to diesel when they get to around 10 years old or more. Then again, no one knows if that'll need to happen to the slightly redesigned Volvo B5LH systems from 2015 onwards
Some of the drivers on my local service do go a bit too fast and sometimes if they allocate a 8.9m dart the speed humps on the 21 and 20 routes are quite large and you end up with a ruined lower back because they drove them like Lewis Hamilton is late for work. 😂
