You can trailhead from either side of the bridge and both direction are a great ride. No matter which side you ride you will have to traverse the paved road to cross the bridge. If you plan to ride late in the day, make the crossing at the beginning of your ride. This is a fun trail with lots of light technical crossing and only a couple of spots where you have to dimount. Roots, rocks, logs, creeks, dips, bridges, and great views, this trail has it all. 7.5 mile trail .25 mile paved.
UPDATE: Keg Creek has been connected to the New Bartram & Wildwood Trail extension. This means that you can ride from the Bartram, Westdam trailhead to Keg Creek and back for a 45~ mile lollipop. Awesome.