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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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REDUCTION AND ANALYSIS OF DATA
The ADAAG set minimum criteria for accessibility of pedestrian facilities throughout the United States. By law, States and local jurisdictions are required to follow ADAAG when constructing and altering any pedestrian facility. In this study, we recognized ADAAG as rule and thus focused on evaluating select design guidelines contained in the AASHTO Guide, wherein discretion may be allowed by a designer. Consequently we have enumerated in this section of the report the applicable AASHTO criteria for each operational characteristic and compared the AASHTO values to the values that were observed for the participants. While the purpose of this research was not to evaluate the AASHTO Guide for the Development of Bicycle Facilities, that document is currently used throughout the United States to set operational criteria for shared use paths, and it considers the bicycle to be the design vehicle. The analyses reveal important information about the physical dimensions, speeds, etc. for various user types and, thus, how well those various users might be accommodated on facilities designed in accordance with the AASHTO bicycle criteria (figure 43). The implications of our findings are covered in the next section, "Discussion." As that section will detail, the bicycle, in many cases, is not the critical design vehicle.
The following discussion focuses on user types for which five or more users were observed in
This study contains some limitations. First, the distribution of participants by user type as shown in the following tables may not be representative of the overall user population on the three trails, nor of shared use paths throughout the United States. For example, the observed proportion of hand cyclists (32 out of 260) is likely higher than their incidence at other times (when promoted "Ride for Science" events were not taking place) (figure 44).
Second, the active participants registered with the data collection team and were aware that they were being observed. Thus, they may have been motivated to perform differently than if they had been in situ participants.
Third, measurements were taken on only individual users, not users traveling side-by-side or one in front of the other. Thus, the design implications discussed in this study pertain only to individual users. For instance, two hand cyclists traveling abreast will require more path width than a solo hand cyclist. Three inline skaters traveling one after the other may require more time to cross an intersection than a single inline skater. Moreover, individuals may behave differently when they are part of a group than when they are traveling alone.
Fourth, the sample sizes by user type varied widely. Among the 260 active users, the largest user type was bicyclists (139). At the other end, twenty-two user types had one user each. Much of the following discussion focuses on user types for which five or more users were observed in this
The AASHTO Guide for the Development of Bicycle Facilities (p. 40) assumes a bicyclist eye height of 140 cm (54 inches) in calculating the minimum length of vertical curve necessary to provide minimum stopping sight distance at various speeds on crest vertical curves or sag vertical curves with overhead sight obstructions.(2) Vertical curves are described in more detail in the Discussion section of this report. Table 7 shows that the mean eye height for bicyclists in this study was 157 cm (62 inches). The 85th percentile eye height for bicyclists was 150 cm (59 inches). In other words, 85 percent of bicyclists had eye heights of 150 cm (59 inches) or greater. Consequently, the AASHTO value seems conservative with the minimum values observed for bicyclists.
Hand cyclists appear to be the critical user for shared use path design of vertical curves, as they had the lowest mean (96 cm (38 inches)) and 85th percentile (85 cm (33 inches)) eye heights. Among equipment types with five or more users, the 85th percentile eye heights were less than 140 cm (54 inches) for users of the following: hand cycles, kick scooters, manual wheelchairs, power wheelchairs, and recumbent bicycles. The low eye heights for scooters may have been due to the users' ages.
Table 7. Eye height.
The AASHTO Guide for the Development of Bicycle Facilities (p. 65) incorporates a bicycle length of 180 cm (72 inches) in its calculations for recommended traffic signal timing.(2) Table 8 shows that the mean length for bicycles was 168 cm (66 inches), and the 85th percentile, 178 cm (70 inches). In other words, 85 percent of observed bicycles had lengths of 178 cm (70 inches) or less. Thus, the values observed for bicycles in particular seem to be consistent with the AASHTO value.
Among equipment types with five or more users, recumbent bicycles appear to be the critical user, as they had the highest mean (190 cm (75 inches)) and 85th percentile (208 cm (82 inches)) lengths. The 85th percentile length of hand cycles also exceeded 180 cm (72 inches). Although only four bicycles with trailers were observed in this study, they had a mean length of 290 cm (114 inches) and an 85th percentile length of 296 cm (117 inches). Thus, the AASHTO value is not sufficiently long for these user types, with potentially serious consequences-see "Refuge Island" in the "Discussion" section of this report.
Table 8. Length.
The AASHTO Guide for the Development of Bicycle Facilities (p. 5) recommends a minimum width of 120 cm (48 inches) for any facility designed to be used by bicyclists.(2) This is based on a typical bicyclist having a width of 75 cm (30 inches) and requiring a minimum of 100 cm (40 inches) of operating space. Table 9 shows that the mean width for bicyclists in this study was 61 cm (24 inches), and the 85th percentile width was 69 cm (27 inches). Hence, the values observed for bicyclists seem consistent with the AASHTO value.
Among equipment types with five or more users, hand cyclists appear to be the critical user, as they had the highest mean (65 cm (26 inches)) and 85th percentile (71 cm (28 inches)) widths. For all user types with five or more users, the 85th percentile width was less than 75 cm (30 inches). Thus, the AASHTO value accommodates above 85 percent of the observed individuals within each user type.
Section 4.2.1 of the ADAAG requires a minimum clear width of 81.5 cm (32 inches) at a point, and 91.5 cm (36 inches) continuously, for single wheelchair passage. All of the "solo" manual and power wheelchair users (i.e., not accompanied by a dog or pulling another wheelchair) had widths of 69 cm (27 inches) or less, and would therefore be accommodated by ADAAG.
Table 9. Width.
The AASHTO Guide for the Development of Bicycle Facilities (p. 65) uses a bicycle acceleration rate of 0.5 to 1 m/sec2 (1.5 to 3.0 ft/sec2) in its equation to determine the minimum green time.(2) Table 10 below shows the observed 85th percentile acceleration rates by user type and distance, in 6.1-m (20-ft) increments. Bicyclists met or exceeded the AASHTO range for distances of up to 12.2 m (40 ft). At greater distances, the acceleration rates were much lower and fell short of the value range. This is an important finding with its relevance explained in the "Discussion" section.
For all distances, recumbent bicyclists had the highest 85th percentile acceleration rates and manual wheelchairs, as expected, had the lowest 85th percentile acceleration rates. For the initial distance traveled, 0 to 12.2 m (0 to 40 ft), hand cycles, manual wheelchairs, and Segways had acceleration rates slower than the lower end of the range used by AASHTO (i.e., slower than 0.5 m/sec2 (1.5 ft/sec2)).
Table 10. 85th percentile acceleration rates (m/sec2).
NOTE: This table includes only active participants.
Table 11 shows the time required for various path users to cover a given distance. This affects calculations for both minimum green time for traffic signals and for pedestrian clearance intervals. The AASHTO minimum green time calculation was performed using the AASHTO 0.5-m/sec2 (1.5-ft/sec2) acceleration rate. The values in the "Pedestrian Clearance" row were
calculated using a walking speed of a constant 1.2 m/sec (4 ft/sec). Again, recumbent bicyclists had the lowest 85th percentile elapsed times and manual wheelchair users had the highest 85th percentile elapsed times. At signalized crossings, pedestrian signals are needed to accommodate the slower travel speeds of manual wheelchair users and pedestrians. For users likely to be operating on the roadway, the critical users for minimum green time would be hand cyclists, as they had the highest elapsed time of the vehicular-type devices. For pedestrian clearance intervals, manual wheelchair users would be considered the critical users; compared to other users, manual wheelchair users take the longest time to cover any given distance.
Table 11. 85th percentile elapsed time (sec).
NOTE: This table includes only active participants.
The AASHTO Guide for the Development of Bicycle Facilities (p. 36) recommends a minimum design speed of 30 km/h (20 mi/h) for shared use paths.(2) Table 12 shows that the mean speed for bicyclists in this study was 17 km/h (11 mi/h), and the 85th percentile speed, 22 km/h (14 mi/h). Thus, the AASHTO value is higher than the speeds observed for most bicyclists.
Recumbent bicyclists appear to be the critical user, as they had the highest mean (23 km/h (14 mi/h)) and 85th percentile (29 km/h (18 mi/h)) speeds. For all user types, the 85th percentile speed was less than 30 km/h (20 mi/h). Thus, the AASHTO value is higher than the speeds observed for most recumbent bicyclists.
The lowest mean speeds were observed for strollers (5 km/h (3 mi/h)) and manual wheelchairs (6 km/h (4 mi/h)). These two user types also had the lowest 15th percentile speeds, 4 km/h (3 mi/h) and 5 km/h (3 mi/h), respectively.
Table 12. Speed.
The speeds of active and in situ participants were compared for each user type. Active bicyclists traveled faster than in situ bicyclists, and this difference was statistically significant. On the other hand, active kick scooters and manual wheelchairs traveled slower than their in situ counterparts, and both differences were statistically significant. For other user types, the observed differences in speed between active and in situ participants were not statistically significant.
NOTES: This table includes both active and in situ participants. S = Significant at the 0.05 level.
- = Not significant.
The AASHTO Green Book (pp. 111-113) recommends a perception-reaction time of 2.5 seconds for motorists.(29) It cites research by Johansson and Rumar, who found a mean reaction time of
0.66 seconds, after collecting data from 321 drivers who expected to apply their brakes.(30) About 10 percent of drivers had reaction times of 1.5 seconds or longer. Also in that study, when drivers did not expect to apply their brakes, their reaction times increased by approximately 1.0 second. Based on that study and other research, the AASHTO Green Book concluded that a value of 2.5 seconds exceeds the 90th percentile perception-reaction time of all drivers and takes into account the additional time required for unexpected braking vs. expected braking.(29) The AASHTO Guide for the Development of Bicycle Facilities (pp. 40-42) uses a perception-reaction time of 2.5 seconds.(2
For this study, the perception-reaction time was measured from when the STOP sign was displayed to when the participant started braking. At the upstream acceleration station, participants were told in advance that at some point along the course they might be presented with a STOP sign, and if so, they were to stop as quickly as is comfortable. (In fact, all participants were asked to stop.) In addition, "dummy" stop stations were set up to reduce the anticipation at a particular location.
Table 14 shows that the mean perception-reaction time for bicyclists was 0.9 seconds. This is consistent with the mean reaction time of 0.66 seconds for motorists, as reported by Johansson and Rumar.(30)
Table 14. Perception-reaction time.
The 85th percentile for bicyclists was 1.3 seconds. Adding 1.0 second to this value results in a value of 2.3 seconds for bicyclists who do not expect to stop. Consequently, the AASHTO value of 2.5 seconds seems adequate for the majority of bicyclists, including those who are not expecting to stop. In fact, with the possible exception of kick scooters (whose riders had an 85th percentile perception-reaction time of 2.3 seconds), the AASHTO value of 2.5 seconds seems appropriate for the majority of other users, including those who are not expecting to stop.
Table 15 shows the braking distance, i.e. the distance traveled from the time that the user initiated braking to the time that the user came to a complete stop, for user groups with five or more users. The calculated friction factor is also shown, using the following equation from the
AASHTO Guide to the Development of Bicycle Facilities (p. 42):(2)
where: S = stopping sight distance, ft
V = speed, mi/h
f = coefficient of friction
G = grade
The first term , is the braking distance (denoted by d), and the second term, 3.67V, is the distance traveled during the perception-reaction time.
In this analysis, G has a value of zero because data were collected on level trail sections. The second term, 3.67V, is not part of the braking distance.
Therefore, the preceding equation simplifies to a braking distance equation:
where: d = braking distance, ft
Rearranging the preceding equation gives:
The friction factor shown in table 15 is that associated with the act of braking. It was calculated by using these values of V and S:
V = 85th percentile speed for that user type, from when the user entered the stopping sight distance station to when the STOP sign was displayed.
S = 85th percentile braking distance for that user type, as observed at the stopping sight distance station
The deceleration rate was calculated as follows:
where: a = acceleration, ft/sec2
For each individual participant, his/her braking distance and braking time were used to calculate his/her deceleration rate. The aggregated deceleration rate for each user type is shown in table 16.
Table 15. Braking distance and friction factor.
Table 16. Deceleration rate.
NOTE: This table includes only active participants.
The implications of these findings are covered in the "Discussion" section below, under the heading "Sight Distance."
The AASHTO Guide for the Development of Bicycle Facilities (pg. 22) recommends a minimum width for bike lanes as 1.2 m (4 ft).(2) Additionally it recommends (pp. 35-36) a minimum width of 3 m (10 ft) for a two-way shared use path (and a wider path is desirable where there is substantial use and/or a steep grade), notwithstanding the procedures given in the Highway Capacity Manual for calculating the number and effects of passing events.(26) In other words, the AASHTO recommendation does not explicitly account for user volumes or passing hindrance resulting from user encounters or overtaking/passing events.
The sweep width was measured as each user traveled through an 8-m (26-ft) section of the course (figure 32). Table 17 shows that the mean max sweep width for bicyclists is 1.0 m (3.3 ft). The 85th percentile max sweep width was 1.2 m (4.0 ft). Hence, the AASHTO values of 1.2 m (4 ft) for bike lanes and 3 m (10 ft) for a two-way shared use path accommodates most users traveling single-file in opposite directions to pass each other, though some only barely.
Table 17. Sweep width (lateral operating space).
Among equipment types with five or more users, inline skates appear to be the critical user. Their mean max sweep width was 1.3 m (4.1 ft), and 85th percentile, 1.5 m (5.0 ft). For all user types, the 85th percentile width was 1.5 m (5 ft) or less. This width slightly exceeds the AASHTO value of 1.2 m (4 ft) for bike lanes. However, the recommended 3 m (10 ft) minimum width for shared use paths is sufficient to accommodate more than 85 percent of the observed individuals within each user type, assuming a two-directional steady linear flow of users traveling single-file on a shared use path.
Section 4.2.2 of the ADAAG requires that the minimum width for two wheelchairs to pass is 1.525 m (60 inches). This assumes that both wheelchair users are traveling in parallel paths to each other and to the edges of the path.
The AASHTO Guide for the Development of Bicycle Facilities (pp. 35-36) recommends a minimum paved width of 300 cm (120 inches) for a two-way shared use path.(2) Table 18 shows that the mean three-point turn width required by bicyclists was 287 cm (113 inches). The 85th percentile turn width was 350 cm (138 inches). Consequently, the AASHTO paved width value accommodates fewer than 85 percent of bicyclists.
Table 18. Three-point turn widths.
Among user types with five or more users, hand cyclists had the highest mean (457 cm (180 inches)) and 85th percentile (541 cm (213 inches)). The 85th percentile of recumbent bicyclists also exceeded 300 cm (120 inches). In fact, 29 out of the 30 observed hand cyclists had three-point turn widths in excess of 300 cm (120 inches). Two of the six observed recumbent bicyclists also had three-point turn widths in excess of 300 cm (120 inches).
According to the AASHTO Guide for the Development of Bicycle Facilities (p. 37), the minimum design curve radius can be calculated by using the following formula:
where: R = Curve radius (ft)
V = Design speed (mi/h)
e = Rate of superelevation (percent)
f = Coefficient of friction
In this study, the trails were flat, so the rate of superelevation was zero, and the formula simplifies to:
This formula can be rearranged as
to calculate friction factors given the speed of each user as he or she traverses curves with specified radii.
It should be noted that what AASHTO refers to as a friction factor is not an actual measurement of the sliding friction of the pavement surface. What it truly represents is the amount of lateral acceleration a user is willing to accept before slowing to a more comfortable speed.
Table 19 of this report shows the friction factors based on 85th percentile speeds. Additionally, the friction factors suggested by AASHTO are provided for comparison. There is a general downward trend in friction factors with increasing curve radii. However, for larger radii the friction factors may level off or even increase. This represents the fact that, at higher radii, users are not slowing substantially from their tangent travel speeds to negotiate the curves (figure 45). Thus, most users could comfortably travel around the larger curves at speeds higher than what was observed in this study. The implications of these results on horizontal alignment are given in the "Discussion" section.
Table 19. Friction factors for different radii, based on 85th percentile speeds.
NOTE: This table includes only active participants.