Can speed be negative

Yes in a form. It can not be literaly -20mph but in deceleration it is. when you slow down a car to a stop that is negative volocity. and volacity is speed+direction. ex: 30mph east. so if your travaling 20mph west then slow to a stop it changes to -20mph west. (until you start going east) but because volacity is a messure of speed then yes it is possible. but not like when approching the speed of light when time slows down, if you go slower than it by so much then it speeds up. (because the speed cant be that form and has to be in directiog)