Iowa Trails 2000

Chapter Four - Section 5
Design Guidelines for Crossings

The greatest potential safety hazard to trail users is when a trail crosses a roadway, railroad, watercourse, or another trail. The best way to increase safety is to increase visibility. It is important that crossings are visible both to trail users and to motorized vehicles. There are two types of crossings: at-grade and grade-separated. Of these, grade-separated crossings are necessary for watercourses, and strongly recommended for high-volume roadways. At-grade crossings are appropriate where motorized traffic volumes are low or local conditions prohibit grade separation.

In any case where a trail will need to cross a roadway, the crossing should be evaluated to determine whether a grade-separated crossing is warranted. This determination will generally be based on roadway capacity, roadway speed, trail capacity, sight distances, and accident history. The FHWA's Manual on Uniform Traffic Control Devices offers guidance on the evaluation of crossings. Primarily in the case of major roadways, a detailed engineering study should be undertaken in the design of the crossing, especially if it is to be at grade, to ensure proper stopping distance, crossing visibility, and sight lines from the trail.

At-Grade Crossings

As discussed above, at-grade crossings should be used when roadway traffic volumes are low, where trails cross roadways at existing traffic signals, or when local conditions restrict the ability to implement a grade-separated crossing.


roadway crossing layout

Wherever possible, trails should cross roadways and railroads at right angles. In cases where trails approach the roadway at a skew, the trail should be routed to achieve a right-angle crossing wherever possible (see Figure 4-27).

Sight Lines

It is important for motorists and trail users to be able to see each other at roadway crossings. A motorist needs to be able to stop in time if a trail user is in the road, and a trail user needs to be able to judge his or her ability to cross the street safely. The AASHTO Policy on Geometric Design of Highways and Streets (Green Book) offers detailed information on determining and planning sight distances at roadway crossings.

Signage, Striping, and Signals

There are three basic components of at-grade crossings.

  • Signage
  • Striping or pavement markings
  • Signals

These three components announce the crossing for both trail users and motorists, and should be employed in varying degrees depending on the functional classification and traffic volume of the roadway to be crossed.

Signage should be based on the FHWA's Manual on Uniform Traffic Control Devices, and be placed outside the recommended clear zones for both trails and roadways. Striping refers to markings painted on the pavement, either on the roadway or on the trail. This includes marked crosswalks and "BIKE XING" lettering. Signals include regulatory traffic lights, such as flashing yellow warning lights, stoplights, or pedestrian crossing signals.

Roadway Crossings

The roadway crossing guidelines described here are divided into five categories based on general functional classifications and roadway volumes. The recommendations for each type of crossing are only a minimum requirement. Each situation must be analyzed in detail to determine whether additional safety signing, striping, or signals are warranted. All roadway crossings should include the following items:

  • Clear sightlines
  • Flush transitions between the trail and the roadway
  • A 3 foot strip of detectable warnings (i.e. truncated domes) as defined in the Americans with Disabilities Act Accessibility Guidelines (ADAAG).

Arterial roads are the largest type of road barring restricted access highways. They typically have a very high traffic volume traveling at very high speeds. They often have more than one lane in each direction. The following elements should be included in crossings of arterial roads (see Figure 4-28):

  • Crossings at signals only
  • Marked crosswalks
  • Cautionary and regulatory signage on trail

Collectors are streets of moderate size that either serve as secondary connections within communities or as primary routes in rural parts of the state. These roadways may have high traffic speed but typically have lower volume than arterial roads. They often have only one lane in each direction, but may be wider in congested areas. The following elements should be included in crossings of collectors (see Figure 4-29):

  • Crossings at signals, at controlled intersections, or mid-block with flashing lights
  • Marked crosswalks
  • Cautionary and regulatory signage on trail
  • Cautionary signage on roadway

crossing layout for arterial roads


crossing layout for collectors

crossing layout for collectors

Residential streets are roadways with low traffic volume and speed. They are found within communities and are designed to serve local residents. The following elements should be included in crossings of residential streets (see Figure 4-30):


crossing layout for residential streets

  • Cautionary and regulatory signage on trail
  • Cautionary signage on roadway
  • Marked crosswalks if trail or roadway traffic volume is high, or if safety concerns exist

Rural roadways serve, as their name suggests, rural portions of the state. They are primarily used as connections to and from agricultural areas. They may have either a paved or a granular surface. They have low traffic volume but traffic may travel at high speeds. The following elements should be included in crossings of rural roadways (see Figure 4-31):

  • Cautionary and regulatory signage on trail
  • Cautionary signage on roadway
  • Marked crosswalks

crossing layout for rural roadways

  • If trail is paved and roadway is not, the trail should be paved across the roadway

Access drives provide connections between residential, commercial, industrial, or institutional properties and an adjacent roadway. They serve only one specific property. Traffic volumes are typically low and travel at slow speeds. Trails adjacent to roadways may cross numerous access drives, depending on the density of the surrounding land use. The following elements should be included in crossings of access drives (see Figure 4-32):


crossing layout for access drivers

  • Cautionary signage on trail
  • Trail may be striped across driveway
On-Road Bicycle Facilities at Intersections

Where bicycle lanes, shared roadways, or paved shoulders intersect with other roadways, they are regulated by the traffic control devices installed at the intersection. This increases safety, as bicyclists are apparent to motorists and are following the same rules. Because on-road bicycle facilities are typically found on the right shoulder, however, right-turn lanes, left-turning bicycles, and ramps of any kind can impact the safe operation of bicycles on the street.

The AASHTO Guide details several options for the accommodation of bicycle lanes at intersections. Bicyclists on shared roadways and paved shoulders, lacking dedicated bicycle facilities along their route, should progress through the intersection according to existing traffic signage and signals.

Railroad Crossings

In many ways, railroad crossings are similar to roadway crossings, except that sight lines for trail users are even more important. As with roadway crossings, trails should cross railroads at right angles, if possible. For many trails, however, this is not likely to be the case, especially for trails within road rights-of way. Bicycles and in-line skaters, in particular cannot cross railroads at a severe angle, because the gap between the pavement and the rail may trap a wheel. The AASHTO Guide details several options for mitigating severely angled railroad crossings (see Figure 4-33).

For paved trails, the railroad track is often set into the surface material. In these situations, the railroad crossings should include a rubberized crossing material, to provide a long-term smooth ride for trail users. For unpaved trails, a ramp leading up to the railroad tie should be developed with a 5 by 5 foot level landing on either side of the track. A rubberized crossing material is also recommended for unpaved trails to minimize maintenance of the railroad crossing.

The opening created by the railroad flangeway should also be addressed if possible. Recently a rubber insert, known as a flangeway filler, has been developed that deflects downward with the weight of a train, but provides a smooth crossing for other users. Currently, flangeway fillers are only available for slower rail speeds.


railroad crossing layouts railroad crossing layouts

Agricultural Crossings

Some trails, especially those on abandoned rail lines, may separate parcels of land owned by the same person. When this land is agricultural, it is necessary for the adjacent landowner to cross the trail to access both parcels. Such a crossing should consider the following recommendations.

  • Sight lines, as based on the AASHTO Green Book, should apply to agricultural crossings. The design speed depends on the trail mode considered, but should adhere to the most stringent standard, as mentioned under "Multi-Use Corridors."
  • Access from the crossing to the adjacent land should be gated, if desired by the landowner, to prevent trail users from perceiving the access road as a spur trail or rest area.
  • In some cases, especially with paved trails, a stronger trail cross section should be constructed in the vicinity of the crossing, so it will not degrade from repeated crossings by heavy equipment.
  • Trails should be signed to indicate the existence of an agricultural crossing.
Crossings of Other Trails

When trails cross one another, users sometimes face specific hazards and require additional information. This is especially true when trails that accommodate different modes intersect. The following recommendations, based on those in the FHWA's Designing Sidewalks and Trails for Access: Best Practices Design Guide, improve safety and provide direction for all users.

  • Offset the trail intersection and create two three-way intersections rather than one four-way intersection.
  • Design the physical connection between the two trails surfaces to be level and smooth.
  • Provide signs at the intersection that clearly indicate the desired direction of travel and the numerous possible destinations (including their distances).
  • Provide sign formats in high use areas, such as audible recording devices, that are accessible to individuals with vision impairments and those who have limited ability to read text.
  • Clearly indicate through signs or barriers the allowed user groups on each trail.
  • Provide objective sign information about the conditions of each trail.
Grade-Separated Crossings

Grade-separated crossings are much safer than at-grade crossings, and should be employed where high traffic volumes exist on the roadway or the trail. There are five types of grade-separated crossings.

  • Roadway underpasses
  • Agricultural underpasses
  • Roadway overpasses
  • Bridges over watercourses and other independent trail bridges
  • Wetland boardwalks
Roadway Underpasses

One method of accomplishing a grade-separated crossing of a roadway or railroad is to build a tunnel underneath the traveled way. This is a good choice if the roadway is elevated, even slightly, because an overpass would be cost prohibitive in these cases. The following recommendations relate to the general dimensions of the underpass. The final design of such a facility requires the consultation of a professional engineer.

  • Minimum width of underpass: trail width plus clear zones. If the underpass must be accessible to an emergency vehicle, the width of the underpass should be a minimum of 10 feet (see Figure 4-34).
  • Wider underpasses are preferred, since they offer a lighter, safer environment for trail users.
  • An underpass should be sloped so it drains out one of the entrances.
  • An underpass should be graded so that exterior surface water is diverted away from the underpass.
  • Good lighting should be provided to increase user comfort and minimize crime.
  • Users should be able to see the other side upon entering the underpass.
  • Long sight lines at the entrance and exit of the underpass should be provided.

 roadway underpass dimensions

Agricultural Underpasses

Agricultural underpasses may be considered when adjacent landowners have the need to move livestock between parcels on opposite sides of the trail. They should be used when trail volume is high and livestock crossing is frequent. Agricultural underpasses are typically large concrete box culverts. The following recommendations relate to agricultural underpasses.

  • Preferred width of agricultural underpass: 6 feet (would allow animals to turn around or travel two abreast) (see Figure 4-35).
  • Preferred vertical clearance of agricultural underpass: 6 feet.
  • The floor of the underpass should be compacted soil.

 agricultural underpass dimensions

Roadway Overpasses

Roadway overpasses may be stand-alone bicycle/ pedestrian bridges designed to carry trail users over major roadways, or alternatively, shoulders or walkways might be retrofitted to carry trail traffic on existing road bridges. It may be less costly to retrofit an existing bridge with a trail facility than to construct a new trail bridge. However, the space on existing bridges is often very limited and the trail facility will be immediately adjacent to heavy traffic. Separate pedestrian and bicycle bridges must be designed with care so that they are accessible to those with disabilities and so that they flow naturally from the trail or street facility and will be used. If the facility will be difficult to use, it is preferable to connect the trail facility to the nearest intersection and cross in that location.

  • Recommended width of independent trail bridge: trail width plus recommended clear zones (see Figure 4-36). In order to reduce cost, recommended clear zones may be reduced on independent trail bridges.
  • Recommended width of trail accommodations on new roadway bridge: trail width plus recommended clear zones (see Figure 4-37).
  • Fencing requirements for independent bridges: fencing should be designed to prevent any objects from falling or being thrown onto the roadway below. Trail bridges may have a fully enclosed cage that maintains the recommended vertical clearance, or they may include fencing that is at least 6 feet high (see Figure 4-36).
  • Fencing requirements for trail accommodations on existing bridge: full-height fencing (as described above) on drop-off side of trail; possible barrier on roadway side of trail, depending on traffic volumes.
  • There should be a flush transition between the trail and the trail bridge.
  • Maximum approach grade for new bridges: 5 percent.
  • Design loading: heaviest vehicle that will cross the bridge, including maintenance vehicles.

independant trail bridge over highway


trail on new roadway bridge

Bridges over Watercourses and other independent trail bridges

The guidelines for bridges over watercourses are much like those for roadway overpasses. The main consideration in designing bridges over watercourses is environmental impact. The following guidelines relate to fencing and to mitigating the potential environmental impacts of bridges. For guidelines on width, approaches, and design loading, see "Roadway Overpasses" above.

  • Reduce the amount of fill in the floodplain by setting bridge abutments as low as possible. The floodplain slope is a good location for bridge abutments, as the trail can progress across the watercourse at a relatively level grade.
  • Avoid setting bridge piers directly in the watercourse. This could disrupt flow and trap debris.
  • Design bridges with as few piers as possible.
  • Fencing requirements for bridges over watercourses: 42-inch fencing on both sides of trail, with a side barrier at trail level to prevent wheels and runners from dropping off the edge of the bridge (see Figure 4-38).

independant trail bridge

  • There should be a flush transition between the trail and the trail bridge
Wetland Boardwalks

In general, trails should avoid wetlands whenever possible. Occasionally, however, for interpretive purposes or out of necessity, a trail may cross a wetland. It is prohibited by federal law to fill the wetland without mitigating the impact elsewhere. Even with mitigation, however, a filled trail corridor would sever the wetland, drastically affecting its hydrology. The trail could be built on an elevated boardwalk for the entire length of the delineated wetland. A boardwalk may follow the same general guidelines set forth for bridges. The following recommendations relate specifically to wetland boardwalks.

  • The wooden trail surface of the boardwalk may cause problems for some user modes. Wood slats should be placed as close together as possible while still allowing for drainage between them.
  • The planks of the boardwalk should be perpendicular to the dominant direction of travel to prevent bicycle tires and wheelchair wheels from becoming stuck in the openings.
  • Understanding that traction by many trail modes on wood (especially wet) is less than on pavement or natural surface, curves and angles should be very slight.
  • Because some wetlands have fluctuating water levels, the trail surface should be located above the normal high water line of the wetland. Floating boardwalks may also be used to accommodate fluctuating water levels (see Figure 4-39).

wetland boardwalk

  • Fencing requirement for wetland boardwalks: 42-inch fencing on both sides of trail, with a side barrier at trail level to prevent wheels and runners from dropping off the boardwalk (see Figure 4-39).