I have an appointment in San Ramon ( near the freeway ) by 5:00pm on a Friday afternoon. I will be in Clayton earlier in the afternoon. What is the latest I can leave Clayton to be sure of getting to San Ramon on time?
Google says it takes 31 minutes without traffic delays.
Easy answer, "depends." Friday afternoon the traffic can be awful and if there is one flat tire or accident the entire place will become a parking lot. I would plan at least an hour just to be safe.
Also get a good street map because if the freeway gets backed up you can find a lot of surface streets that will help you get to San Ramon.
If you have internet access you can check the California Highway Patrol www site for traffic problems, it is the most up to date and minute by minute site you will find. Also, [original link] and google maps will give you traffic speeds along the route so you can see if things are running slow or not.