We’re not going to tell you what the best surface type is for your project, but it should be solid. Here are some things to think about:
Asphalt or concrete surfaces are typically the best option for people who rely on support (wheelchairs, canes, etc) for mobility, and are less likely to cause someone to trip because they are generally smooth and even.
Pathways made out of wood or metal should be durable and slip resistant. Some decking and ramps can be hazardous when wet. Wood can warp and weather. Maintain them well to decrease risk of someone falling.
If brick or cobblestone is used, it needs to be level and free from gaps and bumps. Weather and regular use can cause these surfaces to become extremely uneven over time, creating unnecessary hazards that can then cause people to trip and/or fall. Sometimes the slightest bump (even a half inch) can cause a wheel or foot to get caught and throw off someone’s balance.
If gravel is used, make sure it’s hard-packed. Most wheelchairs, rolling luggage, and strollers aren’t designed to plow through loose terrain. Someone walking in heels can have a difficult time walking on gravel, especially during adverse weather.