When considering, planning, or constructing a bike facility, the first step is to identify the project scope. As more detailed information becomes available on site limitations, construction cost, and funding project impacts, the scope will be refined through the design development process. Basic considerations in defining the scope are facility type (on street, off street, equipment), paving, drainage, structures, and design guidelines used to identify dimensions such as width of paths. The following text provides some basics in identifying the project scope.
When developing the cost of on-street bicycle facilities and shared use paths, the user will need to know how to select construction materials, recommend dimensions, and decide on a path surface. The following is a primer for design consideration of bicycle facilities. Pavement design focuses primarily on shared use paths and other off street facilities. Bicycle facilities on roadways are considered to be a minor part of the structural design of the roadway and are therefore not included as part of the primer. This primer should be used in conjunction with the 1999 AASHTO Guide for the Development of Bicycle Facilities.
On street facilities consist primarily of paved shoulders, wide curb lanes, and bike lanes. All are part of the roadway surface that is also used by motor vehicles. Structural requirements of the road bed including pavement depth are dictated by motor vehicles.
Widths should be increased with higher bicycle use, motor vehicle speeds above 50mi/hr, higher percentage of truck and bus traffic.
Continuous stretches of lane 15 feet (4.5m) or wider may encourage the undesirable operation of two motor vehicles in one lane. Where this much width is available, it is recommended to more seriously consider striping bike lanes or shoulders.
Standards recommend the width be 10 feet or 3 meters for a two-way, shared use path on a separate right of way. Other critical measurements include:
The AASHTO Guide recommends a cross slope of 2%. Other considerations to ensure adequate drainage include:
Proper drainage is one of the most important factors affecting pavement performance. Proper drainage entails efficient removal of excess water from the trail. Surface water runoff should be handled using swales, ditches, and sheet flow. Catch basins, drain inlets, culverts and underground piping may also be necessary. These structures should be located off of the pavement structure.
An overpass, underpass, small bridge, drainage facility or facility on a highway bridge may be necessary to provide continuity to a bicycle path.
The critical dimensions to use in designing underpasses, overpasses, bridges
and tunnels, include:
(a) the minimum width of the trail (usually 10 feet) should be maintained through the structure,
(b) the clear distance of two feet on either side of the trail surface should also be maintained through the structure - otherwise, riders will tend to ride in the center of the trail to stay away from the wall or railing of the structure,
(c) an overhead clearance of 10 feet (8 feet with good horizontal and vertical clearance, good sightlines etc) should be maintained through an underpass or tunnel,
(d) railings, fences or barriers on both sides of a path on a structure should be at least 42 inches (1.1m) high, and where they are higher than this a rub rail should be provided at the approximate handlebar height of 42 inches,
(e) clearances should allow for maintenance and emergency vehicles, as should the strength of the bridge (live loading).
Where it is necessary to retrofit a bicycle path onto an existing highway bridge, several alternatives should be considered in light of what the geometrics of the bridge will allow.
(a) Bicycle path across the bridge on one side. This should be done where
(1) the bridge facility will connect to a bicycle path at both ends; (2) sufficient
width exists on that side of the bridge or can be obtained by widening or restriping
lanes; and (3) provisions are made to physically separate bicycle traffic from
motor vehicle traffic as discussed above.
(b) Wide curb lanes or bicycle lanes over the bridge. This may be advisable where (1) the bicycle path transitions into bicycle lanes at one end of the bridge; and (2) sufficient width exists or can be obtained by widening or restriping.
(c) Use existing sidewalks as one-way or two-way facilities. This may be advisable where (1) conflicts between bicyclists and pedestrians will not exceed tolerable limits; and (2) the existing sidewalks are adequately wide. Under certain conditions, the bicyclist may be required to dismount and cross the structure as a pedestrian.
Because of the large number of variables involved in retrofitting bicycle facilities onto existing bridges, compromises in desirable design criteria are often inevitable. Therefore, the width to be provided is best determined by the designer, on a case-by-case basis, after thoroughly considering all the variables.
Shared use paths in urban and suburban areas often serve travel needs both day and night, for example commuter routes and trails accessing college campuses. Fixed source lighting improves visibility along trails and at intersections, and is critical for lighting tunnels and underpasses. The AASHTO guide recommends using average maintained illumination levels of between 5 and 22 lux, and the Florida DOT recommends 25 as the average initial lux.
Adequate signing and marking are essential on shared use paths, just as they are on streets and highways. Trail users need to know about potential conflicts, regulatory information, destinations, cross streets etc. The Manual on Uniform Traffic Control Devices (MUTCD) provides some minimum traffic control measures that should be applied.
Striping: a yellow center line stripe is recommended where trails are busy,
where sight distances are restricted, and on unlighted trails where night time
riding is expected. The line should be dashed when adequate passing sight distance
exists, and solid when no passing is recommended.
A solid white line may be used to separate pedestrians from bicycle/blading traffic, and solid white edge stripes may also be useful where nighttime riding is expected.
Warning signs: a range of warning signs can be used to inform users that recommended design criteria cannot be met, for example curve radii or grades.
Informational signs: trail users need to know where they are, where they are going, what cross streets they are crossing, how far destinations are away, and what services are available close to the trail. The MUTCD has information on the appropriate signs to use in these instances. Although not in the MUTCD, many trails post signs encouraging uniform trail user etiquette (e.g. give audible signal when passing).
Intersection markings and signs: pavement marking and signs at intersections should channel users to cross at clearly defined locations and indicate that crossing traffic is to be expected. Similar devices to those used on roadways (i.e. stop and yield signs, stop bars) should be used on trails as appropriate.
The AASHTO Guide notes that in addition to traditional warning signs in advance of intersections, motorists can be alerted to the presence of a trail crossing through flashing warning lights, zebra-style or colored pavement crosswalks, raised crosswalks, signals, and neck-downs/curb-bulbs.
The type of surface that will be provided is an important consideration in design. A hard surface, such as cement or asphalt, will generally see cyclists operating at a faster speed than a soft surface, is more expensive to install. A soft surface trail will discourage or prevent in-line skating but may enable horse-back riders to share the trail and is less expensive to install. Factors such as weather conditions and soil types can affect the choice of asphalt, concrete, or crushed rock.
Other considerations of surface material include, terrain, climate, design life, maintenance, cost, and availability. Soft surface materials include earth, grass, bark and wood decking. Hard surface materials include stone, brick, concrete and asphalt. Hard surface materials are preferred for multi-use trails with high bicycle use.
Each surface material type has advantages and disadvantages. Soft surface materials are low cost, but require substantial maintenance and are not suitable for many of the recreational activities today's trails and paths are used for. Hard surface materials, specifically concrete and asphalt, provide years of service with low maintenance.
The key to designing quality pavement surfaces, particularly asphalt surfaces, depends on the following criteria.
Under most circumstances, a 2-3 inch (50-75 mm) thick asphalt concrete top course placed on a 6 inch (150 mm) thick aggregate subbase is suitable for a bikeway pavement structure. While loads on bicycle paths will be substantially less than highway loads, paths should be designed to sustain without damage wheel loads of maintenance vehicles that are expected to use or cross the path. Path width of 12 ft allows service vehicles to travel on the path without encroaching and therefore potentially damaging the edge of pavement and the subbase.
In areas where climates are extreme, the effects of freeze-thaw cycles should be anticipated in the design phase. At driveway crossings of bicycle paths, the highway or driveway should be paved a minimum of 10 feet on each side of the crossing to reduce the amount of gravel being scattered along the path by motor vehicles.
Development of pavement section recommendations assumes a properly prepared sub-grade. The subgrade should be cleared of vegetation and compacted. The subgrade or compacted area should extend at least two feet beyond the edge of pavement.
Bike paths and trails should be constructed to match the existing topography
as closely as possible, however, longitudinal slopes should not exceed five
percent and a cross slope of two percent is desirable to provide adequate drainage
away from the pavement surface. Maintenance
Properly constructed asphalt pavement using an appropriate mix design requires minimal maintenance. Providing proper drainage is also a key to reducing maintenance costs.
Maintenance is generally divided into two categories, preventative maintenance and corrective maintenance. Preventive maintenance is performed on a regularly scheduled basis to improve the life of the pavement and decrease the rate of deterioration. Corrective maintenance is performed to correct a specific pavement failure or distress area.
Normal periodic maintenance, depending on path location, drainage and climate, should include sweeping the trail of debris.
|Soil Cement||Uses natural materials, more durable than native soils, smoother surface, low cost.||Surface wears unevenly, not a stable all-weather surface, erodes, difficult to achieve correct mix.|
|Granular Stone||Soft but firm surface, natural material, moderate costs, smooth surface, accommodates multiple use.||Surface can rut or erode with heavy rainfall, regular maintenance to keep consistent surface, replenishing stones may be a long-term expense, not for steep slopes.|
|Asphalt||Hard surface, supports most types of use, all weather, does not erode, accommodates most users simultaneously, low maintenance.||High installation cost, costly to repair, not a natural surface, freeze/thaw can crack surface, heavy construction vehicles need access.|
|Concrete||Hardest surface, easy to form to site conditions, supports multiple use, lowest maintenance, resists freeze/thaw, best cold weather surface.||High installation cost, costly to repair, not a natural looking surface, construction vehicles will need access to the trail corridor.|
|Native Soil||Natural material, lowest cost, low maintenance, can be altered for future improvements, easiest for volunteers to build and maintain.||Dusty, ruts when wet, not an all-weather surface, can be uneven and bumpy, limited use, not accessible.|
|Woodchips||Soft, spongy surface - good for walking, moderate cost, natural material.||Decomposes under high temperatrue and moisture, requires constant replenishment not typically accessible, limited availability.|
|Recycled Materials||Good use of recyclable materials, surface can vary depending on materials.||High purchase and installtion cost, life expenctancy unkown.|