Doctor McCoy running down a New York City alley
An alley in San Francisco
Posters on the walls in an alley
An alley was a type of road running between or within buildings; that was normally more narrow than a street, usually suitable for pedestrian rather than automobile traffic, although both were common.
Upon arriving in New York City during the 1930s via the Guardian of Forever, James T. Kirk and Spock found a fire escape in an alley with clothing on it that they stole so as to blend in with their 20th century environment. (TOS: "The City on the Edge of Forever")
When Doctor Leonard McCoy arrived in New York in the same manner, he ran down an alley when he saw a homeless man drinking some milk. (TOS: "The City on the Edge of Forever")
In San Francisco in the year 1986, McCoy, Scott and Sulu looked down an alley right next to a saloon that was established in 1881 as they walked past. (Star Trek IV: The Voyage Home)
In 1996, Captain Kathryn Janeway and Commander Chakotay encounter a homeless Braxton in an alley. (VOY: "Future's End")
In 2022, La'an Noonien-Singh and an alternate version of James T. Kirk stood in an alley of Toronto. (SNW: "Tomorrow and Tomorrow and Tomorrow")
In 2259, when Christopher Pike, Spock and La'an Noonien-Singh beamed down to the surface of Kiley 279, it ended up being in an alley that was void of foot traffic. Upon materializing, Pike then asked "Why is it always in an alley?" (SNW: "Strange New Worlds")
In 2266 on Miri, Spock, Fields and Galloway walked under a fire escape in an alley. (TOS: "Miri")
In 2267, James T. Kirk told Spock that adjusting the universal translator to help them communicate with the Companion was "right up [his] alley." (TOS: "Metamorphosis")
Later that year, on the planet Argelius II, Redjac murdered Kara, a female belly dancer, in a foggy alley. Although he managed to frame Montgomery Scott, the chief engineer of the starship Enterprise. (TOS: "Wolf in the Fold")
In 2364, Lt. Commander Data read a detective novel in which a vendor by the name of O'Hagan had set up shop in an alley. He did this to better prepare himself for joining Jean-Luc Picard, the captain of the USS Enterprise-D on an adventure in the holodeck, where he would be playing in a similar story, The Big Good-Bye. (TNG: "The Big Goodbye")
In 2368, when Jean-Luc Picard experienced the life of Kamin because of a mental connection to a Kataan probe, he at first thought that he was abducted, and he left Kamin's home, and walked through the alley outside his residence. (TNG: "The Inner Light")
In 2384, Jankom Pog often played a holo-program that he referred to as a street fighting sim. The simulation featured alleys lit up by neon signs with a biker gang for him to brawl with. Each biker in the alley resembled Doctor Noum of the USS Dauntless. (PRO: "Ghost in the Machine")
In 2401, Worf and Raffaela Musiker pursued the Changeling Titus Rikka down an alley in District Six on M'talas Prime. (PIC: "Seventeen Seconds")