Sounds like someone is reading the first 2 chapters of "Hatchet."
I'd take water, but if you're fixed gear, you might be SOL either way, and a meadow preferable.
Water: snag the gear and flip, face first at 80mph, into the water.
Trees: Hit a tree face first at 80. Ever see a car crash into a tree? Tree usually wins.
Maybe car drivers should try harder to hit the tops of the trees instead of the bottoms.