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Federal Highway Administration Research and Technology
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Publication Number: FHWA-HRT-04-103
Date: October 2004
Characteristics of Emerging Road and Trail Users and Their Safety
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The variety of users on the trails and roadways of the United States has increased dramatically over recent years. Devices such as kick scooters, inline skates, hand cycles, and recumbent bicycles that were rarely seen on shared use paths as recently as 10 years ago are now common. Among the reasons for this shift are the development of new technologies and changing demographics. For example, electric personal transporter devices (e.g., the Segway®) are new technologies that are appearing on paths and roadways around the country. Additionally, coincident with the aging of the American population, the number of people using mobility assistive devices (such as manual wheelchairs, powered wheelchairs, and powered scooters) is increasing.
With the increasing variety of emerging users comes the question of whether we are designing and building suitable facilities. Many jurisdictions throughout the United States have adopted the American Association of State Highway and Transportation Officials (AASHTO) Guide to the Development of Bicycle Facilities as a standard for shared use trail design and other facilities used by nonmotorized travelers. This guide is written with bicyclists in mind, so its recommendations are based on the physical dimensions and operating characteristics of bicyclists. Furthermore, some user groups are petitioning to legally operate on roadways and streets. As this report will document, some users have different characteristics from bicyclists, so facilities designed and built to accommodate bicyclists and/or motorists may not meet the needs of these users.
The Federal Highway Administration (FHWA) recognized this need to accommodate emerging trail users and sponsored this study to better understand their physical dimensions and operational characteristics. This research naturally links the existing capacity methodologies in the Highway Capacity Manual and the FHWA study on "Evaluation of Safety, Design, and Operation of Shared Use Paths"with the design professionals' need for adequate information to design facilities to meet the operational and safety requirements of the increasingly diverse nonmotorized transportation users. It represents a very important step in providing crucial information for the future development of AASHTO's Policy on Geometric Design of Highways and Streets, the AASHTO Guide to the Development of Bicycle Facilities, the AASHTO Pedestrian Facilities Guide, as well as other new design standards.
This report describes the operational characteristics of bicycles and these emerging user types:
Organization of this Report
This report begins with an introduction explaining why this research is needed. The next section discusses potential sources of safety and crash data. Details of the field data collection plan are provided in the third section. The results of the field data collection are then presented, followed by a discussion of the results and their implications for what the design user types should be for each of several design criteria. Next, recommendations regarding design criteria are given. The report concludes with a marketing plan.
Safety/Crash Data Availability
As part of this study, a variety of data sources were examined with regard to their potential for providing information on the safety of emerging road and trail users:
Field Data Collection
In this study, data were obtained by observing shared use path users at seven types of observation stations to generate:
Three data collection ("Ride for Science") events were held in 2003: Pinellas Trail in St. Petersburg, FL (January 25, 2003); Paint Branch Trail in College Park, MD (May 3, 2003); and San Lorenzo River Trail in Santa Cruz, CA (June 13, 2003).
The trail users consisted of both active and in situ (passive) participants. Active participants were those who either were intercepted on the shared use path (via event signage) or responded to the public outreach program and specifically traveled to the location to actively participate in the event. Active participants registered with the data collectors and were given a briefing on the purpose of the study, an overview of the event and course, and safety provisions. They generally progressed through all the data collection stations. In situ participants were those who were on the shared use path just "passing through" the event stations. Thus, at all seven stations, data were collected from active participants. At two stations, speed and lateral operating space, data were also collected on in situ participants. A total of 811 participants (551 in situ and 260 active) were observed at the three events.
This research shows that there is a great diversity in the operating characteristics of various types of road and trail users. To ensure the safe accommodation of emerging road and trail users, their operating characteristics must be considered in the development of design criteria; in some cases, it will be important to use an emerging user instead of the bicycle as the design user.
With respect to sweep width, the critical user is the inline skater, having an 85th percentile sweep widths of 1.5 meters (m) (5.0 feet (ft)). This research only addressed individual users. Users traveling abreast or passing each other may require additional space.
For three-point turn widths, hand cyclists are the critical users, requiring 5.4 m (17.8 ft) (at the 85th percentile level). Several other user types, including bicyclists and recumbent bicyclists, required more than 3 m (10 ft) of width (at the 85th percentile level) to execute a three-point turn.
The design speed of a facility is that speed used to determine the various geometric design features of a facility. It influences many aspects of geometric design. Consequently, it can significantly impact the cost, operational safety, and constructability of a project.
Recumbent bicyclists had the highest observed 85th percentile speeds, 29 kilometers per hour (km/h) (18 miles per hour (mi/h)). This appears to suggest that they may be the appropriate user upon which to set a minimum design speed. However, before coming to such a conclusion, the additional design characteristics of the various users need to be evaluated.
At the other extreme, hand cyclists appear to have the lowest 15th percentile speed, 8 km/h (5 mi/h), of those users who would be expected to operate in the street. Among all user types, strollers had the lowest 15th percentile speed (4 km/h (3 mi/h)).
For horizontal alignment, the critical factor is the point at which users will instinctively decelerate to maintain a comfortable degree of lateral acceleration while traversing a curve. This point is represented by the coefficient of friction used in AASHTO's minimum design radius equations. Most users did not appear to reduce their speeds for radii greater than 15.3 m (50 ft). The exception is recumbent bicyclists, who may have been constrained by even the 27.5-m (90ft) radius.
Stopping Sight Distance
Adequate sight distance is required to provide path users ample opportunity to see and react to the unexpected. The distance required for a user to come to a complete stop, stopping sight distance, is a function of the user's perception and brake reaction time, the initial speed of the user, the coefficient of friction between the user's wheels and the pavement, the braking ability of the user's device, and the grade (all the observations of this research were conduced on flat grades).
The critical trail user for stopping sight distance is the recumbent bicyclist, with a calculated stopping sight distance on wet pavement of 32.7 m (107 ft).
Vertical Alignment-Crest Vertical Curves
The minimum length for a crest vertical curve is a function of the stopping sight distance, the algebraic difference in the upgrade and downgrade grades, the assumed height of an object on the pavement, and the user's eye height.
The recumbent bicyclist would be the critical user for determining the minimum length of a crest vertical curve. Using the 85th percentile stopping distance and eye height, and assuming wet pavement conditions, the minimum length of a crest vertical curve with a difference in grade of 10 percent would be 46.7 m (153 ft) for recumbent bicyclists.
When designing a path crossing of a roadway, refuge islands are frequently provided between opposing motor vehicle traffic flows to allow pathway users to cross only one direction of traffic at a time. The longest likely users, bicycles with trailers, exceeded 2.4 m (8 ft) in length and should be considered the critical users. Two other user groups, recumbent bicyclists and hand cycles, both had 85th percentile lengths in excess of 1.8 m (6 ft).
Signal Clearance Intervals
Roadway users approaching a traffic signal that changes to yellow often cannot stop before the signal turns red; signal clearance intervals allow time to enter and clear the intersection before the cross-street traffic is given a green light. Signal clearance intervals include both the yellow interval and any all-red interval. Signal clearance intervals timed for motor vehicle traffic (typically a maximum of five seconds) provide insufficient time for most users to clear a five-lane (18.3-m (60-ft) wide) intersection. For intersections wider than 8.1 m (24 ft), the kick scooter appears to be the critical user.
Pedestrian Clearance Intervals
Pedestrian clearance intervals allow pedestrians who begin crossing a signalized intersection before the beginning of the flashing DON'T WALK phase to completely cross the street before crossing traffic enters the intersection. Typically, pedestrian signals are timed for walking speeds of 1.2 m/second (sec) (4 ft/sec). The manual wheelchair users evaluated were able to cross intersections within the time provided for an assumed 1.2 m/sec (4 ft/sec) walking speed. A walking speed of less than 1.2 m/sec (4 ft/sec) should be considered in determining the pedestrian clearance time at crosswalks that are routinely used by slower pedestrians, including those who use wheelchairs.
Minimum Green Times
Another signalization criterion is minimum green time. The AASHTO equation addresses three factors in its calculation of minimum green time for signalized intersections: perception-reaction time, acceleration time, and travel time at the design speed. Perception-reaction time is assumed to be 2.5 sec. The AASHTO equation assumes a constant acceleration rate; however, this research shows that the assumption is not accurate. After an initial increase to the acceleration rate, the rate decreases with increasing speed. Rather than produce an equation that compensates for the change in acceleration as speed increases, a table format has been used to represent the distance traveled as a function of time. Further, as many users were on "long" devices (for example, recumbent bicycles had an 85th percentile length of 208 centimeters (cm) (82 inches)), the length of the device is included in the travel distances in table 21 so that the times shown include the time that it takes for the user to accelerate from a stop and completely clear the length of his/her device from the intersection.
Hand cyclists are the critical users to consider when determining the appropriate minimum green times for vehicular devices (table 11, figure 49).
Characteristics of Segway Users
The data from this study provide information on the performance and maneuverability of the Segway and how it might function within shared use paths, sidewalks, and streets. Table 1 shows how the Segway characteristics compare with the design values (for bicyclists) in the AASHTO Guide. The Segway was not found to be the critical user for any design criteria.
Table 1. Characteristics of Segway users vs. AASHTO (bicycle) values.
1 cm = 0.39 inches 1 m = 3.28 ft 1 km = 0.621 mi
The AASHTO Guide for the Development of Bicycle Facilities is the primary reference for designers of shared use facilities and has been adopted as the standard for shared use path design by many jurisdictions around the country. Its standards were developed using the operational characteristics of the bicycle to determine design criteria. While this research is not intended to validate or discredit the AASHTO criteria, comparisons to AASHTO are appropriate because of its status as a national guide.
The data collected for this study reveal that the appropriate design user for shared use paths may vary with respect to design criteria or a facility design element. Table 2 shows design features, AASHTO design values for bicyclists, potential design users, and 85th percentile performance values.
Table 2. Design criteria and potential design users.
1 m = 3.28 ft
It is worth noting that bicyclists (without trailers) do not appear as critical users for any design features. This is a major finding that may have a significant effect on how shared use paths and other components of the U.S. transportation system are designed, constructed, controlled, and maintained.
While additional research is needed to determine which user type should be the basis for specific design criteria, the findings suggest that design guidelines may need to be revised to incorporate the needs of emerging trail users. The results of this study can be used to help design professionals adequately design roadway and shared use path facilities to meet the operational and safety needs of a more diverse group of user types.
The goal of the marketing plan is to outline how to disseminate the information in this report to transportation professionals, trail designers/coordinators, landscape architects, engineers, public works officials, and other professionals and policymakers. The results of this study are being publicized through numerous venues and using different methods. These include slide presentations to Transportation Research Board (TRB) committees and at the Midwest Regional Bike/Ped Conference; a poster session at the TRB 2004 Annual Meeting; new National Highway Institute (NHI) course development (or modifications to existing bicycle and pedestrian facility design courses); and others. Much of this marketing plan has already been implemented.