Where I live there are no real trails, so I ride along the roads all the time. Granted we are out in the country but the roads have next to no shoulder and there is a high school about a mile up the road. There are a few things I've picked up over time...
1) School bus drivers are the WORST! They rarely slow down and never move over. Me and my mustang got DRENCHED one day when they blew at full speed through a big puddle right next to us.
Those buses are loud too!
2) There are some idiots (mostly those with motorcycles and classic cars) who think it is cool to REV their engines as they pass you.
3) People will honk, so be prepared.
4) People will also stop and talk to you.
5) Even if you have worked long and hard at desensitizing your horse I would never take a horse who can be at all spooky along the road. Just when you think you have prepared for every possible scenario, something else unimaginable will pop up. All the horses I ride are pretty much bombproof:)