Yikes, Catrin, did not know there were rattlesnakes in Indiana! 
I think you're probably right about the height and density of vegetation having something to do with the spiderwebs. Also, the width of a trail plays into it. The wider the trail, the fewer spiders can spin a web across it!
I just published a blog post about our last, longest hike in Hot Springs NP if you are interested. This is the one where we ran into a lot of spider webs in the first few miles, which were a part of the trail that gets little use and had definitely not been used that day. Once we passed Jack's Pond, we got to more well-used and wider parts of the trail and didn't have any more issues with webs. No flies that day either. This hike was on June 5, and it was quite hot, but a beautiful hike.
http://travelingtwosome.weebly.com/t...-national-park
I don't think we've hiked since then, actually, though we've done plenty of walking.
Emily
2011 Jamis Dakar XC "Toto" - Selle Italia Ldy Gel Flow
2007 Trek Pilot 5.0 WSD "Gloria" - Selle Italia Diva Gel Flow
2004 Bike Friday Petite Pocket Crusoe - Selle Italia Diva Gel Flow