I've had them in paths, roads, and only twice have I actually had one that was basically against the wall of a building that was on the outskirts of my city. That was when it wasn't fully released though. But roads/paths all can and do get incidents on them.
Note that this isn't a complaint, merely an observation, but I've yet to see one on a 2-lane road in my Oceanic Future city. Meanwhile my Arctic Future city has received incidents on its two-lane roads more than enough times for me to wonder if it's actually not possible for incidents to appear on Aqua Highways. It's merely anecdotal evidence but both cities have had incidents since shortly after the initial rollout and I find it odd that they've never appeared on those roads for me. They both have a similar amount of 2-lane roads as well, so it's not something simple like 'well this city has 4x more roads so of course it's more likely that incidents will appear there'.
Anyway, ever since the full rollout and the increase in rewards that came with it, I've noticed an equal decrease in the frequency of them appearing. It seems that Inno may have gone back to the original version of incidents that was released on beta, more or less. Fewer incidents with larger rewards. Hopefully an improvement for those who don't like sweeping through their city constantly throughout the day!