M25 London Orbital
A motorway, subtly redesigned by Crowley so that seen from above, it took on the appearance of the dread sigil Odegra. The effect of millions of vehicles using it each day and fuming, swearing, and wishing ill on their fellow road-users not only drew millions of souls that bit closer to Satan and hell, it had the same continuous flow effect that you would normally get on a Buddhist prayer wheel.
So when the big day dawned, the cumulative effect of all this negativity effectively sealed off London from the outside world, and incidentally did for Pigbog and the Four Other Riders Of The Apocalypse.
The sight of millions of trapped people in their cars with their electronics chanting
Hail the Great Beast, destroyer of worlds!
for all eternity was something that made Crowley wish he'd never bothered with moving those surveyors' poles a few occultishly significant metres by dead of night. However, a bigger consideration was that he was trapped on the wrong side of the M25 and had to get to Tadfield somehow. Sometimes, a demon can be too clever for his own good...
This is likely based on the authors' own experience of the M25.