He had got up and walked around his chair, he told us, and had thrown the ash of a left-handed cigarette over his right shoulder; he'd show the world that Casey Ryan could and would keep out of gunshot of trouble.
He was rehearsing all this and feeling very self-righteous while he drove down West Washington Street.

True, he was doing twenty-five where he shouldn't, but so far no officer had yelled at him and he hadn't so much as barked a fender.

Down across Grand Avenue he larruped, never noticing the terrific bounce when he crossed the water drains there (being still fresh from desert roads).

He was still doing twenty-five when he turned into Hill Street.
Busy with his good resolutions and the blameless life he was about to lead, Casey forgot to signal the left-hand turn.

In the desert you don't signal, because the nearest car is probably forty or fifty miles behind you and collisions are not imminent.
West-Washington-and-Hill-Street crossing is not desert, however.