My Cannondale Quick 5 came with 700c38s and I have no plans to change them. Some of the area trails have limestone or even blacktop in sections, but in other places there is something more like small gravel with a lot of rounded stone, always loose. And one such spot is on a stretch where they deviated from the rail right-o-way and it has a 12% grade! In some of the Berks County end of the Schuylkill River Trail they have a habit of using whatever is handy to fill washouts and such too. It's not unusual to encounter something more like 3/4" crushed trap rock.
A few descents on the aforementioned 12% grade with my road bike is what sent me out to look at hybrids.