Surface

The trail experience is greatly affected by the choice of surface. Urban trails usually have asphalt or concrete surfaces to withstand heavy trail use and the impact of maintenance vehicles. Hard or sealed surfaces such as these also provide in-line skaters with a place to skate. However, the initial cost of an asphalt or concrete surface will often be significantly more expensive ($125,000 per mile) than a soft or unpaved surface ($40-$50,000 per mile). Paved trails will also likely raise the speed of bicyclists, which may be an issue in areas with heavy pedestrian use.

Rural trails are more likely to be surfaced with crushed limestone as this is usually cheaper and quicker, and is adequate for moderate or low use facilities. However, soft surface trails are not usable by in-line skaters and are more prone to flood or water damage. Some hard surface trails have a soft shoulder that is designed for joggers to use, and in areas with equestrian use, a parallel soft surface trail is recommended where space permits.

Alignment

The most popular and successful trails manage to remain direct without being boring; they avoid sudden or sharp turns and don't meander unnecessarily, and yet they are not completely straight either. The AASHTO guide provides detailed information or horizontal alignment, curve radii and other design elements affecting trail alignment.

Grade

Slopes greater than five percent are undesirable as many bicyclists have trouble climbing this steep of a slope. When traveling down a slope greater than five percent, some bicyclists may exceed the speeds at which they are comfortable or in control. Where terrain dictates, however, slopes may exceed five percent. The AASHTO Guide provides information on grade restrictions and grade lengths (e.g. a ten percent slope is recommended for lengths of only 100 feet), and also on a range of options that can be used to mitigate excessive grades such as signing, added trail width, longer sight lines, and even a series of switchbacks.

Structures

Many rail-trails take advantage of bridges and tunnels that were built to accommodate trains, and are therefore strong enough to support bicycle and pedestrian use. However, some bridges have to be replaced or restored. The AASHTO Guide recommends

• The clear width should be the same as the trail width PLUS the 2 feet of clear space on either side; this provides clearance from tunnel walls or bridge railings and allows clearance from users who may have stopped on the bridge.

• Railings, fences or barriers should be a minimum of 42 inches high.

• Bridges should be designed for pedestrian live loadings and maintenance vehicles.

Intersections

Intersections between paths and roadways are often the most critical issue in shared use path design and require considerable care. Although every intersection is a little different from the next, the AASHTO Guide identifies three basic categories of path-roadway intersection:

midblock crossings: where the trail crosses a highway away from existing intersections
adjacent path: where the trail crosses a highway at an existing intersection
complex: where other configurations exist

The Guide goes on to discuss design issues related to stopping distances, traffic signals, refuge islands and many other factors. Some of the basic principles to apply include:

• Try to ensure trails cross roadways at an angle of 90 degrees, or as close to that as possible, even if this means slightly realigning the trail or roadway.

• Treat intersections consistently so that users can readily identify them as they are approaching.

• Use sound engineering judgement and follow the Manual on Uniform Traffic Control Devices (MUTCD) to determine the type of traffic control device to use.

• Make trail users visible and their movements as predictable as possible.

Roadway Separation

The AASHTO Guide strongly recommends against the development of trails that are immediately adjacent to roadways. This type of facility, that is often little more than a wide sidewalk, encourages wrong-way riding (a leading cause of bicycle-motor vehicle crashes) and makes trail users much less visible to motorists at intersections. Sidewalk trails can also set up conflicts at every driveway or intersection along a roadway.

However, a rail corridor running adjacent to a highway may not suffer from as many negatives as there are likely to be fewer intersections to negotiate and there already may be greater separation between the trail and roadway. The minimum amount of separation recommended is

• five feet of horizontal separation, or

• forty-two inches of vertical separation (provided by a barrier or railing)