There are lots of species of garter snakes with different colors and patterns. But I will say that snake whatever it is non-venomous. Venomous snakes have triangular shaped heads.
Its true that the three (indigenous) Pit Vipers in the U.S. have prominently triangular heads, but I would caution against using that as a singular diagnostic tool for identifying venomous snakes.
Rattlesnakes, Copperheads and Cottonmouth (the three pit vipers) do certainly have a triangular shaped head (and are venomous) but the Coral Snake does not. It is also not a Pit Viper.
Several harmless water snakes have roughly triangular heads or will flatten their head to look that way....but are not venomous.
IF you are not familiar with the other ways to I.D. the venomous snakes in your area....it is best to leave them alone rather than risk a bite.