Some public bus systems will come right to a main entrance. Other systems such as a subway or train station might have a shuttle bus available to bring people to the institution. Some institutions can have 30 to 40 thousand people in one day. These days can be difficult for a person with a disability. Longer waits in lines, more people using auxiliary aids, harder to see exhibits, less of a chance to use interactive features, etc. can make for a long, tiring and sometimes frustrating day.
Most zoos cover many acres.
If you are going to be walking consider planning your route by location of water fountains and benches. Most maps will show location of water fountains. Many free demonstrations occur during each day. Check to see if specific times are scheduled, where the activity is located, and if a reservation is needed. Some demonstrations might be limited by space and require a reservation.