The A666 is sometimes referred to as the Devil's Highway or the Devil's Road because of Biblical associations of its number 666, and its high accident rate on the moors between Egerton and Darwen. Officially, a short length from the M61 Kearsley spur and bypassing Farnworth to central Bolton is called St. Peter's Way.

Because of a crash rate that was three times higher than other motorways in the borough, with 26 vehicle collisions and crashes a year and 40 people injured, road works and other changes were introduced, including the speed limit changed from 70 mph to 50 mph, speed cameras, better safety fencing, banning cyclists from the road, and slip road changes. Finished at the start of 2000, these reduced road accidents by 60%.