I live in hill country often its difficult to tell how fast you're going downhill. People aren't expecting you to be doing the speed limit let alone going faster than the speed limit. Which means you could be the cause of a wreck if you're speeding. Bicycle speedometers can be purchased for less than $ 30. I also have head, brake, tail and signal lights, mirrors, as well as a horn on my bike. Other than a head light for night riding my state (Tennessee) doesn't require the other lights, horn, mirrors or speedometer only reflectors in the rear. However I've got the other lights, horn, mirrors and speedometer for my protection. These are tools to help you manage your ride on the public roadways. This being said being aware and alert is by far the most important thing you can do.