Final answer:
Historically, a ditch's side slope was often 5:1 horizontally to vertically and 7-8 feet deep, but modern design practices recommend side slopes of up to 8 horizontal diameters side-to-side and 15 diameters deep for better safety and drainage.
Step-by-step explanation:
The recommended side slope on ditches adjacent to road shoulders can vary depending on the specific design standards and environmental factors of the location where the road is being designed. Historically, a common rule of thumb for the side slope of ditches was 5 horizontal to 1 vertical (5:1) and 7-8 feet deep.
However, newer studies and design practices suggest that a side slope can be as much as 8 horizontal diameters side-to-side and 15 diameters deep for ensuring better safety and sustainability. The selection of an appropriate ditch side slope is crucial for road safety, maintenance, and water drainage efficiency.