The length you give for the snake seems within the length of garter snakes:
"Common garter snakes grow to be 46 to 137 cm in total length. Males are generally smaller than females and have longer tails. Young common garter snakes are born at 12.5 to 23 cm long and are similar in appearance to the adults. There are many dozens of recognized regional populations of common garter snakes that have distinct color patterns. In some areas there are populations that have a high percentage of entirely black garter snakes. Common garter snakes are similar in appearance to their close relatives, ribbon snakes (Thamnophis sauritus) and Butler's garter snakes (Thamnophis butleri). (Conant and Collins, 1998; Stebbins, 1985; Wechsler, 2001)"
I don't know what part of the country you live in, so it's difficult to know what kind of snakes you have in your locality.