Sir Peter Hendy, recently-knighted Transport for London big cheese, has suggested that Boris Bikes are super-safe because they are big, slow, and have flashing lights.
Firstly, are they actually that safe? Well, the first year produced 6M journeys and 100 collisions. That works out at a rate of slightly less than 17 collisions per million journeys, compared with a rate of a bit more than 20 collisions per million journeys for general cycling in London in the same year. So the safety of the Boris Bikes appears a bit better than the background numbers, but not spectacularly so, and the difference could be explained by many factors, such as user demographics, journey time of day, routes, the hire journeys being shorter and concentrated in the central London area. Another possible explanation is that drivers assume you are clueless if you're on one and make allowances.
It is certainly a stretch to say that a big slow bike is safe. While riding slower may help you avoid some types of collision, it will make you more vulnerable to others. When pulling out into a stream of traffic, you are best advised to accelerate to the speed of the other road users, which is difficult on an under-geared bike weighing 23kg. Riding slowly makes you more vulnerable to left-hooks and badly-timed overtakes by impatient motorists.
The other factor Hendy won't be telling you about is the brakes on the blue barges are crap. Dangerously so in some cases, in my experience.
As for flashing lights, most riders in London use such lights after dark. The lights on the Boris Bikes are quite low-powered and mounted low down at the back. They are better than nothing for sure, but of very limited benefit in daylight, and a lot worse than a well-positioned modern high-powered set-up at night.
The one fatality involving a cycle-hire bike was on TfL's CSH#2, recently branded "an accident waiting to happen". And that fact pretty much sums up cycling safety in London. It really matters very little what kind of bike you ride, when the roads are as woefully dangerous as they are. London cyclist KSIs rose by 18% last year, a number that clearly indicates TfL is not doing its job properly. To suggest that it's the fault of cyclists not using lights is a bit of an insult really. Ironically, the one solitary factor that likely has benefitted cycle safety over the past 15 years is the invention of the high-intensity LED light, but the failure of TfL and London boroughs to implement safe cycle routes, the cynical prioritization of traffic flow over safety, and the failure to police the roads properly has more than offset any benefit and meant that KSIs have resumed an upward trajectory.