ive read every answer here. and none of them reflect actual experience riding a bicycle and getting into multiple accidents on them. i have. i have also SEEN multiple accidents happen.
RIDING ON CITY STREET IN MANHATTAN AND BROOKLYN ; the reality is that the biggest determining factor of how bad your injuries are is HOW YOU FALL OFF YOUR BIKE AND WHAT YOU FALL INTO. it is that simple.
and for this reason, my experience reflects the fact that that higher speeds ensure you fall off your bike in a manner that is FAR LESS CONTROLLED AND MORE SLOPPY. thus, higher speeds ensure your 'fall' is worse, which is entirely the opposite of the thesis that your fall is better if you are faster because you can 'roll'. thats utter nonsense.
if you are going at a slower speed you have a few more microseconds or milliseconds to adjust your body and torso and DIRECTION of your fall. in particular, you put your hands out in front to shield your head, and reflexively try and protect your head. apart from those reflexes, you control the 'wobble' of your bike in a fall that doesn't occur smoothly--many falls occur due to a GRAZING of the bicyle hitting something from the side lightly, including pedestrians, a light knock from a car, hitting a hard pothole, sliding over leaves or a grating that destabilizes your balance . in that type of accident you have like 1/3 of a full second as you try and stabilize your handlebars to no effect. however, if you are going slower the result is the wobble lasts longer and you can sometimes control the direction of the fall, or even turn a 'fall' into a 'slide out' which i have done many times. this means you fall down on your side but in a relatively controlled manner which results in scrapes and scratches and maybe a bruise but nothing broken and no major hard impact as if you are thrown onto the ground.
also in relatively common 'dooring' situations where are a vehicle door opens directly in front of your path, you have just a bit more time to swerve away from the window of the door infront of you, and fall away from the door as opposed to falling through it with your entire body. more importantly at slower speeds, you have a better chance of being able to stop short by slamming your brakes without an accident.
furthermore, there is the obvious reality that hittting an object , any object, at higher speed results in more force upon impact.
finally, the most overlooked reality is that going at a slower speed allows you to avoid FAR MORE accidents and allows others to avoid getting into accidents with you, particularly pedestrians who may otherwise not see you because they are not paying attention at all, or because you aren't paying attention to them.
if there is ONE and only ONE rule for cycling other than 'pay attention at all times' it is , RIDE SLOWLY. it is NOT 'wear a helmet'. no. ride slowly.
the best sped for a bicycle is under 14 miles an hour. at 12 miles an hour, you are basically just strolling around speed on a bike. In my experience most bicycle accidents occur between cyclists and pedestrians running into the street , or between cyclists and vehicle Doors, or between cyclists and potholes/uneven streets or hidden obstacles, slippery things on the street. for all of these most common causes of falls, speed greatly affects the outcome of both the likelihood of the accident occurring, and the results of the accident.