All great advice but one other piece to the formula is the horse's natural inclination to either stand its ground and just "get on with with" or spook/bolt, spook/dip/spin and give you change for a dime.
I happen to have one of each - lol
Once upon a time some moron in an 18-wheeler purposely let his jake brake off right beside my "just get on with it" horse. All Duke did was flip the guy off with his ear and keep going -- he is the horse in my avatar.
He has also gone nose-to-nose with a P.O.'d male llama that spit in his face. I said "get him Duke" and his 14.3H self backed that llama down without blinking an eye. My 16.1H dip & spin horse nearly had heart failure over that llama. I was on a 300+ organized ride both times and ended up having to hand walk the dip & spin guy back to the trailer because he was so blown by the whole chain of llama events
Spook & Bolt and Dip & Spin are both perfectly fine to go down the state highway as long as Duke is with them. They won't miss a beat or flick an ear. Duke is also their strong alpha-dominant leader in the pasture so could probably lead them over the cliff and they would follow
But, alone and under the wrong conditions, one would be side-passing the whole way and the other would plant his hooves on the ground and refuse to move until I get off and lead him.
So, along with the other advice that's been given, it is crucial to know your horse's Level of Common Sense, reaction to things, and of course how to stay seated if she does do some spectacular Airs Above the Ground when a semi or a pack of Harley Davidsons comes whizzing at her.
There's not one thing wrong with getting off and hand-walking as long as it can be done without getting hit by a car:)