A good horizontal trail alignment should provide adequate stopping sight distances for the design trail speed at the approaches (figure 33). An ideal bridge approach, such as a straight approach, would allow off-highway vehicles (OHVs) and bicycles to see oncoming traffic. Curves can be constructed adjacent to the bridge, but vegetation may have to be thinned to improve visibility. Bridges constructed perpendicular to the stream are the shortest structures and usually cost less, but may cause safety problems on the approaches because of inadequate sight distances and a tight turning radius, especially when the trail parallels a stream with brushy banks before making an abrupt 90-degree turn onto the bridge (figure 34).

Figure 33—This bridge is on a good alignment that allows OHV riders to
see oncoming vehicles.

Figure 34—This alignment causes a blind spot for oncoming traffic.

Vertical trail alignment is also important. Bridges with a slight grade will shed water (figure 35). Bridges at the low point of a sag curve in the alignment may have maintenance and safety problems because gravel and debris will collect on the bridge deck or water will pond and freeze there (figure 36). Less efficient alignments are acceptable when you wish to maintain well-vegetated banks or high stream sinuosity, preserve large trees, or protect wildlife habitat. Straightening stream channels or modifying channel alignments is not recommended and requires complex hydraulic and geomorphic investigations.

Figure 35—This bridge has a slight grade, allowing water and debris to run
off the bridge deck.

Figure 36—Gravel collects on the deck of this bridge because it is in the
low point of a sag curve.