Chapter 4— Designing Trail Elements
Safe shared-use trails follow engineering principles that are similar to those used for highways, including adequate sight distance and alignment. With careful design, safe trails don't have to be minihighways—they can adhere to professional standards and still be esthetically pleasing. A single trail corridor can include many design considerations, requiring flexibility on the part of designers. Because each situation is unique, appropriate solutions require sound judgment by the designer, adherence to applicable legal requirements, and sensitivity to local conditions, preferences, and needs.
It is helpful to understand trail structure and the terms that describe it. Figure 4–1 illustrates some common trail corridor terms.
The Student Conservation Association (SCA) uses Lightly on the Land: The SCA Trail Building and Maintenance Manual (Birkby 2006) as a field guide for trail construction. The manual covers basic techniques, from building with timbers to rock construction and environmental reconstruction.
When speaking about trails, it is helpful to use common terminology. This guidebook uses the following definitions:
- Transportation corridor—The larger alignment
of a trail, which may include other modes of
transportation; for example, a multimodal
transportation corridor between two attractions
that has separate trails for stock and bicycles and
a road for motor vehicles.
- Trail corridor—The zone that includes the trail
tread and areas immediately above and to each
side. The edges of single-tread trail corridors
generally are the same as the trail's clearing
width plus its vertical clearance. Multiple-tread
trail corridors include the trail clearing width and
vertical clearance for all the treads. Sometimes
trail corridors include more land than is needed
to accommodate the trail tread and clearance.
- Trail tread or tread—The travel surface of the
- Trailbed—The tread plus base materials.
- Trail clearing width—The space to each side
of the trail tread that is cleared for trail users.
Usually, there is an uphill and a downhill
- Trail vertical or trail overhead clearance— The space over the trail tread that is clear
of obstructions. For riders, this clearance is
sometimes referred to as vertical shy distance.
- Trail clearing limit—The area over and beside
a trail tread that is cleared of trees, limbs, and
other obstructions; often the edges of the trail
- Trailway clearance—The trailbed plus the area to either side that is needed to accommodate construction cuts and fills.
A single trail system can give trail users choices, including scenic variety, different trail lengths, or more than one challenge level. Trails with loops let trail users travel new ground the entire way.
Loop trails allow more miles of trail in smaller areas and avoid the extra traffic of out-and-back—or linear—trails. Elongated loops with cross trails (figure 4–2) allow trail users to select their own trails. An interesting variation contains stacked loop trails, which resemble the links in a chain. A common approach is designing the closest loop to appeal to the greatest number of trail users and to be the easiest to travel. Succeeding loops provide additional length or more challenge.
Trail users' travel speeds differ, and it is important to vary the trail length. Design horse trails no shorter than 5 miles (8 kilometers)—preferably longer. It takes 1 to 2 hours for most equestrians to ride an average 5-mile trail. The length of many day-use trails ranges from 5 to 25 miles (8 to 40.2 kilometers). The best trail systems include a variety of routes that allow rides of 2 to 3 hours, a half-day, and a full day or more. Provide reasonable access to stock water. When practical, the Forest Service (1991) recommends providing water at intervals of no more than 10 miles (16.1 kilometers) and informing visitors if water is not available within this distance. In areas that experience very hot weather, consider locating water sources at 5- to 6-mile (8- to 9.7- kilometer) intervals.
Provide reasonable access to stock water. When practical, the Forest Service (1991) recommends providing water at intervals of no more than 10 miles (16.1 kilometers) and informing visitors if water is not available within this distance. In areas that experience very hot weather, consider locating water sources at 5- to 6-mile (8- to 9.7-kilometer) intervals.
Making the Loop
The Pennsylvania Trails Program (1980) suggests day-use loop trails of 15 to 20 miles (24.1 to 32.2 kilometers) for riders, with an inner loop of 7 to 10 miles (11.3 to 16.1 kilometers) for half-day trips. They recommend providing vehicle access points with adequate parking near overnight stops to allow riders to bring in food and water for stock. The authors note that pedestrians may find all-day equestrian loop trails too long.
Baughman and Serres (2006) recommend horse trails with multiple or single loops that include a variety of scenery and terrain, and an open gathering area. They also recommend trail lengths of 5 to 25 miles (8 to 40.2 kilometers).
Calculating trail distances and trip times is easier if you know the average speed of a trail animal. Horses and mules have different gaits and speeds, depending on breed, training, and physical condition. The speed also depends on the animal's size, trail conditions, topography, size of the riding group, and experience level of the rider.
The average speeds of the most common horse gaits on relatively flat ground are:
- Walk—About 2.5 to 4 miles per hour (4 to 6.4
kilometers per hour), about as fast as a person
- Trot—About 8 miles per hour (12.9 kilometers
- Canter or Lope—About 12 miles per hour
(19.3 kilometers per hour)
- Full Gallop—About 20 to 30 miles per hour (32.2 to 48.2 kilometers per hour)
Most recreation trail users ride their animals at a walk on trails, or combine a walking gait with periods of trotting or cantering, averaging between 4 and 6 miles per hour (6.4 and 9.7 kilometers per hour). Keep in mind that many riders stop along the trail to socialize or enjoy the setting, slowing their average time. Some riders train for endurance rides (figure 4–3)—fast athletic events that cover 50 or 100 miles (80.5 or 161 kilometers).
Mounted riders can see farther than trail users on the ground. This added height helps others see the rider. When approaching the crest of a hill, a trail user should see the head of another trail user on the other side of the hill before reaching the hill's crest. Riders training for endurance races and other trail users that travel at increased speeds require plenty of sight distance to avoid collisions. Downhill travelers need more stopping distance than uphill travelers. Curves in the trail reduce the sight distance; in such cases, trim vegetation along the curve. Design trail curves for appropriate speeds and sight distance to prevent conflicts, considering individual site conditions. The large group of riders shown in figure 4–4 requires a long sight distance to give them time to react.
Sight distance in areas with low development is most critical when trail users encounter approaching bicyclists or riders (figure 4–5). It is often customary for other trail users to yield to horses and mules. To do so, trail users need adequate warning and space. When two horses meet, passing is difficult. Frequently, horses heading uphill take precedence. In some areas, time is used to separate trail users. For example, on the Holland Lake Trail to the Bob Marshall Wilderness in Montana, incoming traffic has the right-of-way until noon, when the preference switches to outbound trail users. Local custom often determines who has the right-of-way. There are no fixed rules that apply nationwide.
People sometimes confuse the terms sight distance, sight line distance, and sight stopping distance. Sight distance and sight line distance—or sight line—usually refer to how far a person can see along an unobstructed line of sight. Sight stopping distance usually takes into consideration the time it takes a traveler to see something, react to it, and stop safely.
View from the Saddle
There are different ways to determine sight distance on trails.
- For trails on small properties, Melvin Baughman
and Terry Serres (2006) recommend a minimum
sight distance of 50 feet (15.2 meters) with 100
feet (30.5 meters) preferred. Provide 100 feet of
sight distance at road crossings.
- On horse trails in Pinellas Park, FL, Orth-Rodgers and Associates (2002) recommend sight
distance of 100 feet (30.5 meters) forward and
- On roads and some trails, especially trails
that intersect with motorized traffic, sight
and stopping sight distances are subject to
guidelines established by AASHTO. Many agencies incorporate AASHTO guidelines into
their own standards, sometimes by reference.
AASHTO publishes numerous guidebooks
that cover highways, roads, roadsides, bridges,
bicycle and pedestrian trails, and other related
subjects. Some AASHTO publications are listed
in Appendix C—Helpful Resources.
- In the United Kingdom, The Highways Agency (2005b) calculates stopping sight distance using rider eye heights, 4.9 to 8.9 feet (1.5 to 2.7 meters) off the ground. This range allows children on ponies as well as adults on larger stock to see, react, and stop in time. Distance calculations must include additional traffic factors, such as the speed of other trail users.
Vegetation that encroaches on tread width and overhead clearance is more than a nuisance for trail users—it can entangle users and gear. Trim or remove vegetation and other obstacles—such as boulders—from this area (see figure 4–1) so trail users can more easily avoid plants that have prickly seeds, thorns, and pointed branches. Periodically providing larger cleared areas for turnouts gives trail users space to move off the tread for breaks or to allow others to pass. Keep in mind that the weight of leaves can cause deciduous tree branches to bend 1 to 2 feet (0.3 to 0.6 meter) in summer and snow can cause evergreen trees to bend in winter, reducing the overhead clearance (Baughman and Serres 2006).
Trail clearance varies by trail use and setting. Table 4–1 shows a general range for clearing widths and overhead clearance on single-track horse trails. Tread width is discussed later in this chapter. Appropriate clearing width depends on the site. For example, on shared-use bicycle/pedestrian trails, AASHTO (1999) recommends at least 2 feet (0.6 meter) of graded width on each side of the tread. A distance of 3 feet (0.9 meter) is preferred from trees, poles, walls, fences, guardrails, and other obstructions. On Forest Service pack and saddle trails in the Northern Rockies, the trail clearing width is 8 feet (2.4 meters) and the trail vertical clearance is 10 feet (3 meters).
Baughman and Serres (2006) of the University of Minnesota Extension recommend a clearing width of 8 feet (2.4 meters) on one-way trails or trails with light use. They recommend a clearing width of 12 feet (3.6 meters) on two-way trails or trails with heavy use.
On level terrain, trails are cleared an equal distance on either side of the tread centerline. Using the previous Forest Service trail example with a 2-foot (0.6-meter) tread, the clearing width would be 3 feet (0.9 meter) on either side of the tread, for a total cleared width of 8 feet (2.4 meters). It is unnecessary to remove all the vegetation from the side of the trail. Instead, consider leaving vegetation or objects less than 30 inches (762 millimeters) tall. The cleared area—also called load clearance (see figure 3–11)—accommodates items tied to saddles, such as picnic articles, sporting gear, or very full saddlebags, but it's also useful when two trail users must pass on a narrow trail. The concept applies to urban and rural areas if the trail does not already have substantial shoulders or horizontal clearance. Consult the land management agency's guidelines.
On moderate to steep side slopes, extensive travel along the lower—or outer—edge of the tread can cause the tread to fail. A log cut nearly flush with the trail's downhill trail edge will encourage travelers to move toward the center of the tread. Rocks, limbed trees, and other natural materials near the lower edge of the tread also help guide traffic back to the center. Obstacles left as guide material on either side of a trail can interfere with loads and can catch a rider's legs or stirrups. Be sure to leave load clearance as described previously. Experienced trail stock may adjust their position on a trail tread to avoid contact with encroaching objects—less experienced stock may not.
To compensate for guide material left near the downhill edge of the trail, cut and remove material for a greater distance from the centerline on the uphill side. When slopes are steeper than 50 percent, consider providing additional horizontal clearance for logs or protruding branches. For example, on the 2-foot (0.6-meter) wide Forest Service trail cited earlier, extend the clearance 6.5 feet (2 meters) from the centerline. This would mean clearing 5.5 feet (1.7 meters) beyond the edge of the tread. This added clearance is particularly necessary for packstock because a horse may shy away from any object near its head. Widen trails cut through solid rock on steep hillsides to provide load clearance. Also, widen the trail base along a precipice or other hazardous area. Using a 2-foot (0.6-meter) Forest Service trail as an example, hazardous trail segments would be widened to 4 or 5 feet (1.2 or 1.5 meters) for safety. Wider treads also provide safe passing areas. Planning this flexible clearance takes some thought and may be difficult for inexperienced trail construction crews. Meander the clearing edges so the trail looks natural.
Low vertical clearance is a potential safety hazard for riders when stock need maneuvering space. Vertical clearance for physical barriers, including bridges, underpasses, and vegetation, should extend at least 10 feet (3 meters) above the tread. Vertical clearance of 12 feet (3.6 meters) is recommended. Increasing the vertical clearance, especially on engineered structures, can be quite costly, and designers must exercise good engineering judgment.
Cut tree and shrub branches back to the tree trunk or to the vegetation's stem. Don't cut all vegetation back exactly the same distance. In some cases, some slightly encroaching vegetation may help slow trail users down. During construction of new trails, minimize plant disturbance. Using the least obtrusive tool to do the job helps accomplish this goal. When highly valued or rare plants cannot be trimmed and must be removed, consider relocating them. On public lands, follow guidelines for sensitive plant species that require extra protection.
It is important to know which vegetation is toxic to stock to avoid routing trails nearby. If toxic plants can't be avoided, the next best choice is to remove them. If toxic plants can't be removed, use signs that identify toxic plants adjacent to trails, especially in highly developed or high-use areas. See Chapter 7—Planning Recreation Sites for more information on toxic plants.
Roadside Use of Native Plants (Kartesz and others 2000) addresses preserving and restoring native plants. The State-by-State section lists native, endangered, and noxious plants. Additional resources also are included. The document is available at http://www.fhwa.dot.gov/environment/rdsduse.
Tread is the actual travel surface of the trail, where the hoof meets the surface. Tread is constructed and maintained to support the designed trail use and may or may not be paved. Most trail construction involves establishing solid, obstacle-free tread that stays in place. A good job of locating, constructing, and maintaining tread discourages trail users from creating their own paths.
No national standards establish the width of shared-use trails. Determining the best trail width is site-specific and depends on many factors, including the types of trail users and their needs, the level of development, the setting, land availability, jurisdictional requirements, safety, potential conflicts, local expectations, and maintenance concerns.
To accommodate their natural stride, horses and mules require a tread that's at least 1.5 to 2 feet (0.5 to 0.6 meter) wide. The trail animal and rider require about 4 feet (1.2 meters) of unobstructed width, and packstock with loads require a minimum unobstructed width of 5 feet (1.5 meters). If stock frequently carry bulky items, the suggested minimum clearing width is 6 feet (1.8 meters).
Tread width also varies by the number of incorporated lanes—or tracks. A single-track tread forces trail users to travel single file. They must move off or to the side of the trail when meeting or passing others. A double-track tread allows trail users to travel two abreast, meet, or easily pass. Single-track treads vary from 1.5 feet (0.5 meter) wide in wildland areas to 8 feet (2.4 meters) or wider in urban areas. Double-track treads are often 5 to 6 feet (1.5 to 1.8 meters) wide if there is plenty of clearance on each side to allow passing. This is a common configuration for moderately developed trails in rural settings. In highly developed areas, double-track treads frequently are 8 to 12 feet (2.4 to 3.6 meters) wide to meet the needs of all trail users. Trails should be wider in areas with heavy shared use.
Flexible Tread Width
On single-track trails with low, but steady use, the Pennsylvania Trails Program (1980) recommends a minimum tread width of 2 feet (0.6 meter) for stable soils and 3 feet (0.9 meter) for poorer soils. Where there are frequent encounters between stock and other trail users coming from opposite directions, the minimum suggested tread width is 6 feet (1.8 meters). In areas with steep dropoffs or other hazards, the recommended width is 8 feet (2.4 meters), which allows stock to pass each other safely.
In areas with low development, trail users usually have fewer encounters with other users, and the trail tread can be narrower. To allow proper use and to reduce animal impacts, horse trails with low levels of development require at least 1.5 to 2 feet (0.5 to 0.6 meter) of tread width. Narrower trails force stock, particularly packstock, to step off the tread. The outer edges of a wildland trail generally receive the greatest impacts from packstock and wildlife. The suggested tread width for horse trails is summarized in Table 4–2. Narrow single-track treads require trail users to move to the side when others pass. Design cleared areas or wide spots to accommodate this practice. Double-track treads may need additional width near walls, fences, or other obstacles. Highly developed trails often have to be wider to accommodate higher traffic volumes and multiple trail user groups. The preferred tread width on shared-use trails depends on who is doing the sharing. The guidelines in table 4–2 apply to most nonmotorized shared-use situations except those involving bicycles—which require additional considerations.
Not all equestrians are found in the saddle—some drive single animals or teams pulling carriages, wagons, carts, sleighs, or other conveyances. Stock that pull carts require tread width that accommodates the vehicles. Single-horse runabout carts (figure 4–6) require a tread width of 4 to 5 feet (1.2 to 1.5 meters), and those pulled by teams of two or more animals require even more. Figure 4–7 shows common dimensions for a runabout cart pulled by a standard-sized driving horse. Fourwheeled conveyances pulled by a team of animals are longer and wider than single-horse runabout carts. Other trail users passing in either direction require adequate space to go around. The minimum preferred tread width for a team of animals is 12 feet (3.6 meters). Consult carriage manufacturers or local equestrians for more details.