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Chapter Four - Section 3

Trail Design Guidelines: Use Modes

While it is most common for use modes to be combined on trails or within corridors, Iowa Trails 2000 discusses each mode to ensure that the needs of various users are thoroughly considered. When combining use modes, the guidelines for each mode should be consulted and the most stringent should be used (see "Multi-Use Corridors"). The modes considered include hiking/walking, bicycling, in-line skating, equestrian, snowmobiling, off-highway vehicles (OHVs), and motorcycles (canoe trail designation is covered later). Each of these use modes is described below, and guidelines are set forth relating to the following design considerations.

  • Clear Trail Width refers to the width of the traveled part of the trail that is free of protruding objects and obstacles, such as trees and overgrown vegetation (see Figure 4-5).
  • Clear Zones refer to the area on each side of the trail between the traveled surface and any obstructions, such as trees, walls, or fences (see Figure 4-5).
  • Vertical Clearance refers to the height above the trail which is free from protruding objects and overhead obstructions, such as tree branches or bridges (see Figure 4-5).
FIGURE 4-5: TRAIL DIMENSIONS

diagram showing trail dimensions

  • Trail surface refers to the type of surface on the traveled part of the trail, such as asphalt, concrete, granular, or alternative. Surface quality is affected by tread obstacles, such as roots or rocks, and by any openings such as gaps and grates located within the trail surface.
  • Drainage refers to techniques used to move and keep water off the trail and trail embankment.
  • Alignment refers to the horizontal curvature of the trail.
  • image referring to definition for alignment

  • Profile refers to the vertical curvature of the trail.
  • image referring to definition for profile

  • Edge protection refers to any protective barrier designed to separate the trail from its surrounding environment, such as a fence or curb. As a general rule, curbs should not be less than 4 inches in height. Other types of edge protection are discussed, where appropriate, under each trail mode.
Topics covered in later sections include:

These design guidelines are meant as general recommendations. Many of the design considerations listed above will be impacted by local conditions, such as topography, right-of-way width, and intensity of use. Each trail project is unique, and while these guidelines should be employed wherever possible, deviations may occur.

Hiking/Walking Trails

Pedestrian facilities can take several forms. Hiking/walking trails, sidewalks and pedestrian trails provide different user experiences for pedestrians.

Hiking/walking trails, covered in this section, are facilities used exclusively by pedestrians, and are typically found in natural areas. They offer a low-impact means of allowing pedestrians to come in contact with the natural environment. Hiking/walking trails are used by a variety of people with a broad range of abilities, skill levels, and desired experiences, and should be designed to accommodate all persons. New and reconstructed trails should be made as accessible as possible while maintaining the essential character of the resource. Furthermore, all trail amenities, such as restrooms, drinking fountains, and picnic tables should comply with the ADA accessibility guidelines. Because of their rustic nature, the guidelines for hiking/walking trails are very general, and trail design will be primarily determined by site conditions.

Clear Trail Width
  • Recommended clear trail width for hiking/walking trails: 4 feet (this may be reduced based on site conditions and desired trail experience) (see Figure 4-6).
  • Hiking/walking trails should include widened areas at regular intervals to allow users to pass one another. These widened areas should be at least 5 feet by 5 feet.
  • In urban or suburban locations, hiking/walking trails should be set back at least 5 feet from any roadway curb.
Clear Zones

Hiking/walking trails do not typically require clear zones, since users are moving at relatively slow speeds. In natural areas, underbrush should be trimmed so that it does not hang over the trail edge or obstruct the traveled way.

FIGURE 4-6: TRAIL DIMENSIONS FOR HIKING/WALKING TRAILS

diagram depicting vertical clearance for hiking/walking trails

Vertical Clearance
  • Hiking/walking trails should maintain an 8-foot minimum vertical clearance (see Figure 4-6). If the hiking/walking trail is used by cross-country skiers during the winter months, the average snow level should be added to the 8-foot minimum.
Trail Surface
  • Hiking/walking trails may be surfaced with wood chips or crushed stone, or may be made of compacted earth. In any case, the surface should be firm and stable. It should be noted, however, that wood chips are not considered an accessible surface.
  • In wet areas a boardwalk is recommended (see "Wetland Boardwalks").
  • Any tread obstacles, such as rocks or roots, imbedded into the trail surface should be less than 2 inches.
  • Any openings within the trail surface, including on bridges, should not permit passage of a 0.5-inch diameter sphere and should be perpendicular to the dominant direction of travel.
Drainage

Because users of a hiking/walking trail will come in direct contact with the trail surface, drainage is very important. Natural surface trails can become watercourses during heavy rains, causing severe erosion. The following methods effectively move water off the trail.

  • In flat areas, the trail should be cross-sloped or crowned at approximately 2 percent.
  • Where a trail is benched into a slope, a swale on the uphill side should be considered to catch water before it crosses the trail.
  • Culverts may be necessary to move water under the trail.
  • Disturbed areas should be seeded and mulched or sodded to prevent erosion.
Alignment

Users of hiking/walking trails can navigate even the tightest of turns. Alignment guidelines are not necessary for hiking/walking trails.

Profile

It is recommended that no more than one-third of the total trail length for a hiking/walking trail exceed 8.3 percent. In addition, the following guidelines should be followed:

  • Trail grade may be 5 percent or less for any distance.
  • Trail grade may be 8.3 percent for a maximum distance of 200 feet.
  • Trail grade may be 10 percent for a maximum distance of 30 feet.
  • Trail grade may be 12.5 percent for a maximum distance of 10 feet.

The trail grade between the maximum grade segments should return to 5 percent for a minimum distance of 5 feet to allow resting opportunities for people who have difficulty traveling over sloped surfaces.

If, due to local topography, the trail would be steeper than the above recommendations permit, switchbacks should be used to lessen the overall slope.

Edge protection

Edge protection is not required on a hiking/walking trail; however, if provided it should be at least 4 inches. Pedestrians with vision impairments tend to adjust their obstacle detection to a slightly higher level on hiking/walking trails because of all the small obstacles contained within a natural trail surface. Edge protection that is at least 4 inches high is much more likely to be detected.

Pedestrian Trails

Pedestrians are typically accommodated with other trail users such as bicyclists and in-line skaters, within a multi-use corridor. In some cases, however, pedestrians may be accommodated on an exclusive trail, as a means of separating pedestrians from faster moving bicyclists and in-line skaters.

Where pedestrian use is expected, facilities should be accessible to a variety of people with a broad range of abilities, skill levels, and desired experiences, and should be designed to accommodate all persons. New and reconstructed trails should be made as accessible as possible while maintaining the essential character of the resource. Furthermore, all trail amenities, such as restrooms, drinking fountains, and picnic tables, should comply with the ADA accessibility guidelines.

Pedestrian trails, unlike hiking/walking trails, are designed for a more formalized trail experience. Whereas hiking/walking trails may be quite rugged, pedestrian trails are typically designed for more leisurely walking on finished surfaces.

Clear Trail Width
  • Recommended width for pedestrian trails: 5 feet.
Clear Zones

Because of the relatively slow speed of pedestrians, clear zones are not necessary.

Vertical Clearance
  • Pedestrian trails should maintain an 8-foot minimum clearance. If the hiking/walking trail is used by cross-country skiers during the winter months, the average snow level should be added to the 8-foot minimum.
Trail Surface

Pedestrian trails, as discussed above, will almost always exist in conjunction with non-motorized multi-use trails. Their surface, therefore, should be the same as that used for the adjacent multi-use trail. Where pedestrian trails occur alone, they may be asphalt, concrete, or granular. Whenever possible, the surface of a pedestrian trail should be smooth and free of tread obstacles. Any openings imbedded into the trail surface should not permit passage of a 0.5-inch diameter sphere and should be perpendicular to the dominant direction of travel.

Drainage
  • Pedestrian trails should have a 2 percent cross-slope.
Alignment

Users of pedestrian trails can navigate even the tightest of turns. Alignment guidelines are not necessary for pedestrian trails.

Profile

It is recommended that no more than one-third of the total trail length for a pedestrian trail exceed 8.3 percent. In addition, the following guidelines should be followed:

  • Trail grade may be 5 percent or less for any distance.
  • Trail grade may be 8.3 percent for a maximum distance of 200 feet.
  • Trail grade may be 10 percent for a maximum distance of 30 feet.
  • Trail grade may be 12.5 percent for a maximum distance of 10 feet.

The trail grade between the maximum grade segments should return to 5 percent for a minimum distance of 5 feet to allow resting opportunities for people who have difficulty traveling over sloped surfaces.

Edge protection

Edge protection is not required on a pedestrian trail; however, if provided it should be at least 4 inches.

Sidewalks

Sidewalks are pedestrian facilities primarily used in cities and towns. They are typically designed for pedestrians only, and should not be used by bicyclists. Sidewalks typically offer pedestrian connections within a community, and are, therefore, an important component of local pedestrian planning. Guidelines for this type of facility are found in the handbook "Local Community Planning for Bicyclists and Pedestrians," (Iowa Trails 2000).

Bicycle Trails

There are extensive guidelines that have been established for bicycle facilities. Bicycles, however, are unlikely to ever enjoy exclusive use of a trail facility. In most cases, bicycle trails will also accommodate pedestrians and in-line skaters on a single paved treadway.

Because bicycles typically travel at higher speeds than pedestrians, trail geometrics are a major consideration. The AASHTO Guide is an invaluable resource when designing bicycle trails. The guide gives detailed information on alignment and profile layout and design.

Clear Trail Width
  • Recommended width for two-way bicycle trail: 10 feet (may be increased to 12 feet depending trail traffic) (see Figure 4-7).
  • Recommended width for one-way bicycle trail: 6 feet (Separated one-way trails in the same corridor should have a minimum 2-foot median between them).
Clear Zones
  • Bicycle trails should maintain a minimum 2-foot graded area on each side of the trail, graded at a maximum slope of 6:1 (see Figure 4-7).
  • Bicycle trails should maintain a minimum 1-foot buffer zone between the edge of the graded clear zone and any fixed objects such as signs or trees. On bridges this guideline does not apply (see Figure 4-7).
FIGURE 4-7: TRAIL DIMENSIONS FOR BICYCLE TRAILS

diagram depicting vertical clearance for bicycle trails

Vertical Clearance

Bicycle trails should maintain an 8-foot minimum vertical clearance (see Figure 4-7).

Trail Surface
  • Asphalt or concrete are the preferred surfaces for bicycle trails.

The surface of a bicycle trail should be smooth and free of tread obstacles. In some cases, granular surfacing may be used as an interim solution. Granular trails can be difficult to maintain, and can be harder on bicycles than paved trails. In addition, granular surfacing eliminates use of the trail by in-line skaters. Any decision to use granular surfacing for bicycle trails should be carefully evaluated.

Drainage

It is very important that bicycle trails are well drained. Standing water on the trail will adversely affect the trail surface and decrease the life and quality of the trail.

  • Bicycle trails should not exceed a uniform cross slope of 2 percent (see Figure 4-8). Crowning of the trail at 2 to 3 percent is acceptable, but may be more difficult and costly to construct (see Figure 4-9).
  • Where a trail is benched into a slope, a swale on the uphill side should be considered to catch water before it crosses the trail (see Figure 4-10).
  • Culverts may be necessary to move water under the trail.
  • Disturbed areas should be seeded and mulched or sodded to prevent erosion.
FIGURE 4-8: TRAIL CROSS SLOPE

diagram depicting trail cross slope

FIGURE 4-9: CROWNING OF A TRAIL

diagram depicting crowning of a trail

FIGURE 4-10: TRAIL WITH DRAINAGE SWALE

diagram depicting trail width drainage swale

Alignment

The design of bicycle trail alignment can be as complex as roadway design. Many factors must be taken into consideration, including design speed, the surface type, and sight lines. The AASHTO Guide and "Minnesota Bicycle Transportation Planning and Design Guidelines" offer detailed information on alignment and superelevation. In general, a typical curve radius for a bicycle trail will be approximately 100 feet.

Another issue to consider when designing a trail’s alignment is visibility on horizontal curves, which is based on stopping sight distance. Stopping sight distance refers to the amount of time it would take a user to stop once an obstruction has come into view. As a general rule, the distance a user can see along the trail should never be less than the distance it would take that user to stop. Procedures for determining stopping sight distance are detailed in the AASHTO Guide and should be applied to both alignment and profile.

Profile

The profile of a bicycle trail is also a major consideration which requires detailed analysis and design. Issues to consider when designing a trail’s profile include steepness (or overall grade of the trail) and stopping sight distance (discussed above). The following recommendations are for general planning purposes only. Final trail design requires more detailed analysis based primarily on the AASHTO Guide.

  • Maximum recommended grade for bicycle trails: 5 percent.
  • Grades on bicycle trails steeper than 5 percent are possible, but should be restricted to distances as indicated in the AASHTO Guide.

Stopping sight distance applies to vertical curves (hills) just as it does to horizontal curves. This consideration is especially important on downhill sections, as speeds will be higher. As described above, the AASHTO Guide is an invaluable resource for detailed trail design, and should be consulted during the final design process.

Edge Protection

Edge protection, typically in the form of fencing, is required on bicycle trails only in areas where safety is a concern. Such safety considerations should be evaluated in detail during the final design of the trail. If fencing is provided, it should be at least 42 inches high. Some possible situations where fencing might be warranted include:

  • Locations where the land on either side of the trail drops off steeply.
  • Locations where sharp curves may cause users to lose control and leave the trail.
  • Locations where adjacent uses, such as railroad tracks or active industry, may cause a threat to trail user safety.
  • Bridges (see "Grade-Separated Crossings").

Where fencing is included, rub-rails should be installed for the safety of bicyclists and wheelchair users. Rub-rails should be installed at ground level and at the general level of an adult bicyclist’s handlebars.

In-line Skating Trails

In-line skaters are typically accommodated along with other modes. They will be commonly found along with bicyclists and pedestrians on multi-use trails. In-line skating trails, therefore, can use the standards described for bicycle trails (see ">Bicycle Trails").

On-Road Bicycle Facilities

There is extensive literature relating to guidelines for on-road bicycle facilities. AASHTO and FHWA, as well as many states, offer a wide range of guidelines for various types of bicycle accommodations. There are essentially three types of on-road bicycle facilities: paved shoulders, shared roadways (including wide curb lanes), and bicycle lanes. All on-road bicycle facilities should be designed so bicyclists travel in the same direction as motorists.

Safety is of great concern in the design of on-road bicycle facilities. Conflicts with pedestrians, automobiles, or other bicyclists can lead to serious injury. Poorly maintained pavement, snow build-up and debris can also lead to safety problems. The guidelines listed below are minimum recommendations only, and site-specific conditions may dictate variations for safety purposes.

Clear Trail Width
  • Paved shoulders: minimum 4 feet, to accommodate bicycle use, but refer to AASHTO’s "A Policy on Geometric Design of Highways and Streets (Green Book)" and FHWA’s "Selecting Roadway Design Treatments to Accommodate Bicycles" for recommendations for greater shoulder width, which is desirable where shoulders provide multiple benefits and where motor vehicle speeds exceed 50 miles per hour (see Figure 4-11).
  • Paved shoulders adjacent to guardrails or other roadside barriers: 5 feet.
  • Widened curb lanes: 14 feet of usable lane width (see Figure 4-12).
  • Widened curb lanes on steep uphill segments: 15 feet (continuous wide lanes greater than 15 feet are not recommended, as motor vehicles may use them as two lanes).

 

FIGURE 4-11: PAVED SHOULDER DIMENSIONS

diagram depicting paved shoulder dimensions

FIGURE 4-12: SHARED LANE DIMENSIONS

diagram depicting shared lane dimensions

  • Minimum width of bicycle lanes: 4 feet as measured from edge of roadway, or 5 feet as measured from the face of the curb or a guardrail to the bicycle lane stripe (see Figure 4-13).
  • Desirable width of bicycle lanes: 5 feet as measured from edge of roadway.
  • Minimum width of bicycle lanes adjacent to parking: 5 feet (see Figure 4-14).
FIGURE 4-13: BICYCLE LANE DIMENSIONS

diagram depicting bicycle lane dimensions

FIGURE 4-14: BICYCLE LANE DIMENSIONS ADJACENT TO PARKING

diagram depicting bicycle lane dimensions adjacent to parking

One issue that may impact on-road bicycle facilities is the presence of rumble strips. Occasionally used on roadways with rural sections, they will lessen the usable width of an on-road bicycle facility. Rumble strips "…are not recommended where shoulders are used by bicyclists unless there is a minimum clear path of 1 foot from the rumble strip to the traveled way, 4 feet from the rumble strip to the outside edge of paved shoulder, or 5 feet to adjacent guardrail, curb or other obstacle." (AASHTO Guide, 1999).

Clear Zones, Vertical Clearance, Trail Surface, Alignment, Profile, and Edge Protection

On-road bicycle facilities will normally benefit from design standards required by the roadway itself. Such requirements are sufficient for the bicycle facility. On-road bicycle facilities should only be designated on hard-surfaced roadways.

Drainage

The primary drainage issue to consider regarding on-road bicycle facilities is the existence of roadway drain inlets. Some types of inlet grates may trap a bicycle wheel or send the rider off course. Bicycle-compatible inlets are widely available, and these should be used on all roadways where bicyclists are expected. On rural sections, the cross-slope required by roadway construction is adequate to drain the bicycle facility.

Mountain Bike Trails

Mountain bike trails are typically rugged, off-road facilities. They have far less stringent guidelines than non-motorized multi-use trails, but can accommodate only one type of bicycle. The hallmark of mountain bike trails is the "single track," which is a narrow pathway with many hills and sharp turns. Such facilities can vary greatly in difficulty.

Recently, there has been a surge of people who recreate in off-road wheelchairs that are designed similarly to mountain bikes. However, not every mountain biking trail will accommodate the additional width of off road wheelchairs (approximately 28 to 34 inches). Therefore, trail designers should post objective information about the minimum clear width of the trail, so people who use off road wheelchairs can make informed recreation decisions.

Clear Trail Width
  • Desirable width for mountain bike trails: 2 feet (see Figure 4-15).
Clear Zones
  • Shrubby vegetation should be removed to a distance of 3 feet on each side of the tread. Established trees and grasses may remain (see Figure 4-15).
FIGURE 4-15: TRAIL DIMENSIONS FOR MOUNTAIN BIKE TRAILS

diagram depicting trail dimensions for mountain bike trails

Vertical Clearance
  • Mountain bike trails should maintain an 8-foot minimum clearance (see Figure 4-15).
Trail Surface
  • Preferred surface for mountain bike trails: compacted earth.
Drainage

Without proper drainage, mountain bike trails may become severely eroded. Several options exist for properly draining mountain bike trails.

  • Mountain bike trails should be cross-sloped at 3 to 5 percent.
  • Flexible waterbars or swales should be used to remove water from trails.
  • Special consideration should be given to placement of trails.
Alignment

Alignment of mountain bike trails will primarily depend on the difficulty of the trail to be constructed. In general, the tighter the turn, the more challenging a trail may become.

Profile
  • Maximum overall grade for mountain bike trails: 10 percent. This level of steepness will allow minor increases or decreases in slope to avoid obstacles. Dips and inclines should be built into the trail to provide interest and facilitate drainage.
Edge Protection

Edge protection is not usually required for mountain bike trails. In areas where safety is of great concern, fences with a minimum height of 42 inches should be installed.

Equestrian Trails

Trails designed to accommodate horses have a great deal of flexibility in design. The most important consideration for equestrian trails is the surface, which should be designed to reduce injuries to animals and riders. The placement of obstacles is also a key issue for designing equestrian trails. Some people with mobility impairments are able to travel by horseback but are not able to walk a horse around obstructions. Therefore, equestrian trails should not require the rider to dismount to avoid obstacles while on the trail. In all design elements, the safety of the horse and rider is paramount.

Clear Trail Width
  • Desirable tread width for equestrian trails: 4 feet (see Figure 4-16).
  • Desirable cleared trail width for equestrian trails: 8 feet (see Figure 4-16).

Tread width refers to the actual traveled surface of the trail. Cleared trail width refers to the areas where underbrush, branches, and other obstructions have been removed. In most cases, there will be little difference between the two, as riders will use the entire cleared area, especially when passing in opposite directions.

FIGURE 4-16: TRAIL DIMENSIONS FOR EQUESTRIAN TRAILS

diagram depicting trail dimensions for equestrian trails

Clear Zones

The cleared trail width listed above includes adequate clear zones for equestrian use.

Vertical Clearance
  • Equestrian trails should maintain a minimum vertical clearance of 10 feet (see Figure 4-16).
Trail Surface
  • Equestrian trails should have a surface of uncompacted natural material.
  • Equestrian trails should be free from brush, stumps, logs, large rocks, and other obstructions that may injure horses.
Drainage

Areas where standing water is likely should be drained by sloping the trail or installing ditches.

Alignment

Horses can maneuver almost any corner, and can travel at low speeds. Therefore, no alignment guidelines are necessary for equestrian trails.

Profile

Because equestrian trails are used by animals carrying a significant amount of weight, trail grade is an important consideration.

  • Maximum grade for equestrian trails: 10 percent.
  • Maximum grade for shorter slopes (100 feet) on equestrian trails: 20 percent.
  • Switchbacks should be used for surmounting slopes greater than the above parameters.
Edge Protection

Edge protection is not usually required for equestrian trails. In areas where safety is of great concern, fences should be installed.

Snowmobile Trails

Snowmobile trails are unique among the trail modes considered in Iowa Trails 2000 because their use will only take place in winter. This seasonal dependency necessitates some unique design considerations. In addition, snowmobiles are capable of high speeds, increasing the need for safety through trail design. As with all motorized trails, signing should be used to warn non-motorized users of the predominate use mode. In some situations clearly indicated dual trails can be indicated for the safe sharing of a corridor by motorized and non-motorized users.

Clear Trail Width
  • Desirable groomed surface for one-way snowmobile trails: 8 feet (see Figure 4-17).
  • Desirable groomed surface for two-way snowmobile trails: 10 feet.
  • At sharp corners or unusually rugged terrain, the trail should be widened to accommodate grooming equipment and provide user safety.

The groomed surface refers to the area which is free from branches, large rocks, brush, stumps, and other obstructions that would create an uneven and unsafe surface even when the trail is covered with snow.

FIGURE 4-17: TRAIL DIMENSIONS FOR SNOWMOBILE TRAILS

diagram depicting trail dimensions for snowmobile trails

Clear Zones

  • Snowmobile trails should maintain a 2-foot clear zone on each side of the groomed surface (see Figure 4-17).
Vertical Clearance
  • Snowmobile trails should maintain at least 10 feet of vertical clearance above the average snow level to accommodate grooming equipment (see Figure 4-17).
Trail Surface

Many snowmobile trails are enjoyed by other trail users during the summer months. In these situations, the surface should be designed according to the needs of the additional user. If the trail is not used during the summer, a variety of surfaces are possible because the trail will be buried with snow for snowmobile use. The surface should be relatively flat and free from obstructions as listed above.

  • Snowmobile trails may exist on an otherwise unprepared surface, provided that stumps, brush, and other obstructions are removed. Snowmobile trails within road rights-of-way demonstrate this type of surface.
  • Snowmobile trails may exist on crushed stone surfacing.
  • Snowmobile trails may exist on wooden bridges or boardwalks when crossing watercourses or wetlands.
  • Placement of snowmobile trails on asphalt surfaces should be avoided, as studs will cause damage to the asphalt. When implementing a snowmobile trail along with an asphalt trail, a natural surface corridor should be provided and clearly marked for snowmobile use.
Alignment
  • Minimum forward visibility for snowmobile trails: 50 feet.
  • Minimum radius for snowmobile trail curves: 25 feet.
  • Where hazards exist (such as a steep drop-off) near a curve, the trail should be superelevated.
Profile
  • Maximum slope for snowmobile trails: 12 percent.
  • Maximum grade for shorter slopes (100 feet) on snowmobile trails: 25 percent.
  • Snowmobile trails should ascend steep slopes at right angles to the contour lines (directly up the fall line). Ascending such slopes at angles could cause sliding of snowmobiles and slope erosion.
Edge Protection

Edge protection is not usually required for snowmobile trails. In areas where safety is of great concern, fences should be installed.

Other Points to Consider
  • Water crossings: Even though ice may be in place for much of the snowmobiling season, water crossings without bridges are not acceptable as part of a snowmobile trail.
  • Exposure: In order to extend the snowmobiling season, trails should be placed, wherever possible, to retain snow cover. Tree lines, woods, valleys, and north-facing slopes are areas that tend to retain snow, and these areas should be sought out for snowmobile trails.
  • Signage: The Iowa Department of Natural Resources has developed uniform signage for snowmobile trails. The DNR’s signage scheme should be used for all snowmobile trails. These signs should be installed before the first snowfall and removed in the spring.
  • Maintenance: Snowmobile trails require a significant amount of maintenance, since winter storms can take their toll on trailheads, signage, and the groomed trail itself. Such maintenance issues should be considered during the initial planning stages of the project.
  • Noise abatement: There is the potential for disturbance from snowmobile noise. For this reason, snowmobile trails should be placed as far as possible from residential areas. Other noise abatement possibilities include placing the trail behind existing vegetation or within valleys. In addition, sound monitoring and enforcement should be initiated to ensure that machines do not exceed the legal limits.

Off-Highway Vehicle Trails (3- and 4-wheeled)

As with snowmobiles, off-highway vehicles (OHVs) are capable of high speeds, and safety is a primary consideration in the establishment of design guidelines. OHV trails may exist as either a nodal or linear facility, with nodal facilities offering looping trails within one designated area or park, and linear facilities offering connections between riding parks, communities, and support services. As with all motorized trails, signing should be used to warn non-motorized users of the predominate use mode. In some situations clearly indicated dual trails can be indicated for the safe sharing of a corridor by motorized and non-motorized users.

The Iowa Department of Natural Resources has recently established a policy regarding the acquisition and development of OHV parks (nodal facilities). This policy is included in Appendix E.

The following guidelines generally hold true for trails in both nodal and linear facilities.

Clear Trail Width
  • Recommended width for a one-way OHV trail in a wooded area: 5 feet (see Figure 4-18).
  • Recommended width for a two-way OHV trail in a wooded area: 8 feet.
  • Recommended width for a one-way OHV trail in an open or grassy area: 4 feet (see Figure 4-19).
  • Recommended width for a two-way OHV trail in an open or grassy area: 8 feet.
  • Trail width on switchbacks or in areas with steep side slopes should be increased by 6 to 20 inches.
  • On sharp curves, trail width should be increased by 1 foot.
FIGURE 4-18: TRAIL DIMENSIONS FOR ONE-WAY OHV TRAILS IN WOODED AREAS

diagram depicting trail dimensions for one-way OHV trails in wooded areas

FIGURE 4-19: TRAIL DIMENSIONS FOR TWO-WAY OHV TRAILS IN OPEN AREAS

diagram depicting trail dimensions for two-way OHV trails in open areas

Clear Zones
  • OHV trails should maintain a 1-foot minimum clear zone on each side of the trail (see Figures 4-18 and 4-19).
Vertical Clearance
  • OHV trails should maintain a vertical clearance of at least 9 feet (see Figures 4-18 and 4-19).
Trail Surface
  • OHV trails should have a natural surface.
  • OHV trails should be placed on soils that are resistant to erosion. Sandy soils should be avoided. County soil survey maps should be consulted to determine the best location for an OHV trail.
  • The OHV trail surface should be free of logs, large rocks, stumps, brush, and other obstructions, unless a more challenging experience is desired. In such a case, some obstacles may be left in place.
Drainage

Improper drainage on OHV trails can lead to rutting and severe erosion. Trails can be drained by using changes in grade or rolling drain dips. Waterbars should be used as a last resort, as they increase maintenance costs.

Alignment
  • Minimum radius for curves on OHV trails: 10 feet.
  • OHV trails should be widened slightly at curves for safety reasons (see "Clear Trail Width" above).
Profile
  • Variety in grades for OHV trails is recommended, as it increases the challenge and desirability of the trail, and facilitates drainage.
  • Minimum slope for OHV trails (for drainage purposes): 2 percent.
  • Maximum continuous slope for OHV trails: 8 percent.
  • Maximum grade for shorter slopes (100 feet) on OHV trails: 15 percent.
Edge Protection

Edge protection is not usually required for OHV trails. In areas where safety is of great concern, fences should be installed.

Other Points to Consider
  • OHV parks: Facilities specifically designated for OHV use can offer great challenge and variety. Such parks are typically designed with a system of loops, beginning at a trailhead and possibly offering several loops of different ability levels. OHV parks are likely to be shared by motorcyclists, so loops should be planned for these users, as well.
  • Erosion: To reduce the potential of erosion, OHV trails should avoid unstable soils and provide adequate drainage, especially on steep slopes and hillsides.
  • Noise abatement: OHVs may reach noise levels significantly higher than allowed by the Code of Iowa. Natural buffers such as hills, ridges, and existing vegetation can help to mitigate noise impacts. To reduce noise conflicts, OHV parks should have regular sound level monitoring to ensure all OHVs comply with the Iowa Code.

Motorcycle Trails

Motorcycle trails are very similar to OHV trails in that they both accommodate motorized recreational vehicles. These two trail modes often use the same facilities, the only exception being motorcycle-only trails located in OHV riding areas (see "Other Points to Consider" above). The following guidelines relate only to variations in trail width, alignment, and profile associated with motorcycle-only trails. For all other trail elements, guidelines for OHV trails should be followed. As with all motorized trails, signing should be used to warn non-motorized users of the predominate use mode. In some situations clearly indicated dual trails can be indicated for the safe sharing of a corridor by motorized and non-motorized users.

Clear Trail Width
  • Recommended width for a one-way motorcycle trail in a wooded area: 3 feet.
  • Recommended width for a two-way motorcycle trail in a wooded area: 6 feet (see Figure 4-20).
  • Recommended width for a one-way motorcycle trail in an open or grassy area: 2 feet (see Figure 4-21).
  • Recommended width for a two-way motorcycle trail in an open or grassy area: 6 feet.
  • Trail width on switchbacks or in areas with steep side slopes should be increased by 6 to 20 inches.
  • On sharp curves, clear trail width should be increased by 1 foot.
FIGURE 4-20: TRAIL DIMENSIONS FOR TWO-WAY MOTORCYCLE TRAILS IN WOODED AREAS

diagram depicting trail dimensions for two-way motorcycle trails in wooded areas

FIGURE 4-21: TRAIL DIMENSIONS FOR ONE-WAY MOTORCYCLE TRAILS IN OPEN AREAS

diagram depicting trail dimensions for one-way motorcycle trails in open areas

Alignment
  • Minimum radius for curves on motorcycle trails: 6 feet
  • Motorcycle trails should be widened slightly at curves for safety reasons (see "Clear Trail Width" above).
Profile
  • Variety in grades for motorcycle trails is recommended, as it increases the challenge and desirability of the trail, and improves drainage.
  • Minimum slope for motorcycle trails (for drainage purposes): 2 percent.
  • Maximum continuous slope for motorcycle trails: 12 percent.
  • Maximum grade for shorter slopes (100 feet) on motorcycle trails: 30 percent.