Devil's Island is a huge traffic island in the middle of a vast inter-city highway complex in Mega-City One. Criminals who commit serious crimes are placed there as it is surrounded by computer-controlled lorries which travel past at 200 miles an hour, day and night.
Here is what Judge Dredd: The board game has to say about it:
This huge traffic island in the middle of a vast intercity megway complex gained its name when it was turned into a prison for lawbreakers who had committed serious crimes. Escape is impossible because the island is surrounded by computer-controlled juggernauts which thunder past in a continual stream at a minimum speed of 200 miles per hour..
- Judge Dredd - The Game of Crime-Fighting in Mega-City One: Umpty Candy Card Album
- Judge Dredd: Judge Whitey, 2000AD Prog 2