Выражение встречается в следующем контексте:

Lock your alley gate and prevent burglary

A number of alley gates have been installed in the East Wallasey area in the interests of preventing house burglary and other crime. When the gates are installed and locked the gates can prevent burglary by as much as 72%.

The presence of alley gates also prevents other crime and disorder problems from occurring such as youth disorder, drug use and fly-tipping.

http://www.merseyside.police.uk/html/wirral/initiatives/general/03-04-alleygate.htm
Заранее спасибо