Not necessarily a bad idea. My city has a long main street with really big city blocks, but there are also a few major stores the next block over that would realistically take 5-10 minutes to walk around to. A couple of alleyways have been turned into walkways - basically just painted up, cobblestoned and had cafe store fronts opened onto them with outdoor seating. They're only between buildings that are a couple of storeys high, so they're not dark.

If they've got pretty high pedestrian traffic then this indicates you've screwed up your city planning and should correct it.

whidbey: May your alleyway never be the dodgy feces grease and urine obstacle course that is Pioneer Square near Yesler.

You forgot to mention the rats.

///If they clean up the alleys, where are the rats going to hang out?

ekdikeo4: Walk around downtown San Francisco.  Even the alleyways where they keep their dumpsters are clean, lit, and even decorated in places.

Chinatown, the Richmond, and the Sunset however, keep their bins of rotten food right out front on the street where it belongs.

CruJones: My neighborhood has alleys between all the houses, I have no idea why. Currently two coyotes are roaming them. No one drives down them, and you put your trash on the street. It's just ten feet of wasted space between houses, that the city has to keep from becoming overgrown with weeds, bushes, etc.

Local building ordinances may prohibit structures from being built within 10' of each other, to restrict the chance of fires spreading from property to property. If you feel adventurous, go to the appropriate planning agency and find out what the real property lines are for your home... you might own half that alleyway.

"ALLEY! ALLEY!"