In OD&D, a 1-2 result on a d6 indicates surprise. A second d6 roll indicates the distance at which the surprised group becomes aware of the other party: 1-2 means 10' distant, 3-4 means 20' distant, and 5-6 means 30' distant.
I thought of an alternative method that's nearly equivalent and slightly faster.
Roll a d8 to check for surprise. A result of 1-3 indicates surprise. Multiply that same roll by 10 to get the encounter distance in feet. Since the non-surprise encounter distance is 40-80', that same roll is still good for the encounter distance if there's no surprise.
Using that method, the chance of surprise increases 4%, and the average non-surprise encounter distance goes from 50' to 60'. I think it's close enough to be worthwhile.