Our horses are kept a 5 minute ride from the city. We ride along highways, into town, we rode to the mall once. We ride to the local takeaway for lunch, to the lake for a picnic, i've ridden to my house and friends houses.
Main thing is, know your horse, and know your own reactions. Most accidents I have seen are when the person freaks out or gets nervous and the horse picks up on it. I have stood on a traffic island in the middle of a road while a bus pulled up about a metre in front of my horses nose - I sat there like nothing was wrong and he reached out to sniff the bus - Yep, an Arab :o Lol!
Know how to settle your horse if they get worked up, know how to relax and bluff your way through a scary situation, know how NOT to tense up and grab ahold with reins and legs, know how to divert your horses attention to a different tak is something happens. Basically, have a well trained horse, a capable rider, and know your horse. You can't control what other people do but you can control how you and your horse react.