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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

Figure 43: Photo. The study reveals important information on various users now common on shared use paths. An event staff person is taking measurements of a participant using inline skates. Two children on nonmotorized kick scooters wait nearby. Another event staff person is sitting at a table and writing down the measurements.
Figure 43. The study reveals important information on various users now common on shared use paths.

Study Limitations

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).

Figure 44: Photo. Thirty-two hand cyclists were active participants in this study. Three participants on hand cycles and one participant in a manual wheelchair are waiting for instructions from an event staff person.
Figure 44. Thirty-two hand cyclists were active participants in this study.

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

 

 

Eye Height

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.

USER TYPE N MEAN (cm) STD DEV (cm) 15TH PER (cm) 85TH PER (cm) 95TH PER (cm)
Adult tricycle 2 157 10 162 152 151
Bicycle 139 157 12 167 150 133
Bicycle trailer 4 160 15 170 152 148
Hand cycle 32 96 11 105 85 78
Inline skates 26 168 13 181 164 157
Kick scooter 13 147 20 165 134 114
Manual wheelchair 7 121 6 126 116 113
Power scooter 1 132 NA 132 132 132
Power wheelchair 6 124 5 129 119 120
Power wheelchair + dog 2 117 11 122 112 110
Recumbent bicycle 6 126 11 133 118 110
Segway 4 188 12 196 179 177
Skateboard 3 155 13 164 146 144
Stroller 3 133 49 163 100 84
Other 12 148 16 162 132 121

NOTES:

  1. This table includes only active participants.
  2. AASHTO value for bicyclists = 140 cm (54 inches)
  3. NA = Not applicable because N=1.

Length

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.

USER TYPE N MEAN (cm) STD DEV (cm) 15TH PER (cm) 85TH PER (cm) 95TH PER (cm)
Adult tricycle 2 163 15 156 171 173
Bicycle 139 168 11 163 178 180
Bicycle trailer 4 290 9 283 296 299
Hand cycle 32 181 15 163 195 198
Inline skates 26 39 9 34 41 45
Kick scooter 13 68 5 63 74 76
Manual wheelchair 7 99 15 87 108 119
Power scooter 1 112 NA 112 112 112
Power wheelchair 6 123 13 108 134 138
Power wheelchair + dog 2 119 14 112 126 129
Recumbent bicycle 6 190 18 178 208 217
Segway 4 56 0 56 56 56
Skateboard 3 76 0 76 76 76
Stroller 3 124 40 100 151 164
Other 12 192 61 146 245 289

NOTES:

  1. This table includes only active participants.
  2. AASHTO value for bicyclists = 180 cm (72 inches)
  3. The lengths for inline skaters are their rest lengths. Their lengths while in motion will vary depending on where they are in their stride.
  4. NA = Not applicable because N=1.

Width

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.

USER TYPE N MEAN (cm) STD DEV (cm) 15TH PER (cm) 85TH PER (cm) 95TH PER (cm)
Adult tricycle 2 65 1 65 66 66
Bicycle 139 61 8 55 69 72
Bicycle trailer 4 66 11 59 74 80
Hand cycle 32 65 7 58 71 74
Inline skates 26 52 9 45 62 64
Kick scooter 13 39 6 34 45 46
Manual wheelchair 7 62 4 61 64 67
Power scooter 1 58 NA 58 58 58
Power wheelchair 6 65 4 62 68 68
Power wheelchair + dog 2 130 0 130 130 130
Recumbent bicycle 6 62 8 54 67 69
Segway 4 64 0 64 64 64
Skateboard 3 24 1 23 25 25
Stroller 3 51 6 47 55 57
Other 12 95 30 64 124 132

NOTES:

  1. This table includes only active participants.
  2. AASHTO value for a facility designed to be used by bicyclists = 120 cm (48 inches)
  3. NA = Not applicable because N=1.

Acceleration

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).

USER TYPE N DISTANCE TRAVELED (6.1-M INCREMENTS)
0-12.2 m 12.2-24.4 m 24.4-36.6 m 36.6-48.8 m
Bicycle 122 0.5 1.4 0.2 0.2
Hand cycle 33 0.4 0.8 0.1 0.1
Inline skates 21 0.6 1.4 0.1 0.1
Kick scooter 11 0.5 1.0 0.1 0.1
Manual wheelchair 9 0.2 0.2 0.0 0.0
Recumbent bicycle 6 1.0 1.7 0.3 0.4
Segway 4 0.3 0.9 0.1 0.1
Other 31 0.5 0.9 0.1 0.1

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).

USER TYPE N DISTANCE TRAVELED
12.2 m 24.4 m 36.6 m 48.8 m
Bicycle 122 4.8 7.7 10.3 12.7
Hand cycle 33 6.6 10.6 14.6 17.9
Inline skates 21 4.7 7.6 10.4 12.8
Kick scooter 11 4.7 8.0 11.4 14.8
Manual wheelchair 9 8.1 15.4 22.8 30.0
Recumbent bicycle 6 3.3 6.1 8.5 9.7
Segway 4 4.5 7.8 10.7 13.9
AASHTO calculation   5.2 9.8 11.4 12.8
Pedestrian clearance   10.0 20.0 30.0 40.0

NOTE: This table includes only active participants.

Speed

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.

USER TYPE N MEAN (km/h) STD DEV (km/h) 15TH PER (km/h) 85TH PER (km/h)
Bicycle 367 17 6 11 22
Golf Cart 5 16 4 12 19
Hand Cycle 38 14 7 8 19
Inline Skates 53 16 5 12 23
Kick Scooter 22 12 3 9 15
Manual Wheelchair 6 6 2 5 8
Pedestrian 38 7 2 5 10
Power Wheelchair 12 9 5 5 11
Recumbent Bicycle 24 23 7 18 29
Segway 4 15 2 14 17
Skateboard 10 13 5 8 20
Stroller 7 5 3 4 6
Tandem 3 19 6 15 22
Other 7 14 4 11 17

NOTES:

  1. This table includes both active and in situ participants.
  2. AASHTO value for shared use paths = 30 km/h (20 mi/h)

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.

Table 13. Speed-active vs. in situ participants.

USER TYPE ACTIVE N ACTIVE MEAN (km/h) IN SITU N IN SITU MEAN (km/h) NOTE
Bicycle 114 19 235 16 S
Golf cart 0 - 1 20 NA
Hand cycle 24 15 9 11 -
Inline skates 24 17 25 16 -
Kick scooter 11 12 5 15 S
Manual wheelchair 6 6 6 11 S
Pedestrian 0 - 30 7 NA
Power wheelchair 8 10 2 7 -
Recumbent bicycle 9 21 12 25 -
Segway 4 15 0 - NA
Skateboard 4 13 6 13 -
Stroller 2 3 5 6 -
Tandem 2 17 1 23 NA
Other 1 16 5 13 NA

NOTES: This table includes both active and in situ participants. S = Significant at the 0.05 level.

- = Not significant.
Blank = Mean not calculated because N=0.
NA = Statistical significance not tested because N<2.

Stopping Distance

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.

USER TYPE N MEAN (sec) STD DEV (sec) 85TH PER (sec)
Bicycle 130 0.9 0.7 1.3
Hand cycle 32 0.9 0.6 1.2
Inline skates 20 1.2 0.5 1.4
Kick scooter 14 1.2 0.8 2.3
Manual wheelchair 8 1.1 0.3 1.5
Power wheelchair 6 0.8 0.5 1.3
Recumbent bicycle 6 0.8 0.3 1.0
Segway 5 1.1 0.6 1.5
Other 24 1.3 0.2 1.5

NOTES:

  1. This table includes only active participants.
  2. AASHTO value = 2.5 sec

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)

Equation 2: S equals V squared divided by 30 times the result of f plus or minus G. The result is added to 3.67 times V.     (2)

where: S = stopping sight distance, ft

V = speed, mi/h

f = coefficient of friction

G = grade

V 2

The first term Equation 2: S equals V squared divided by 30 times the result of f plus or minus G. The result is added to 3.67 times V., 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:

Equation 3: d equals V squared divided by 30 times f.     (3)

where: d = braking distance, ft

Rearranging the preceding equation gives:

Equation 4: f equals V squared divided by 30 times S.     (4)

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:

Equation 5: a equals the result of 2 times d divided t squared.     (5)

where: a = acceleration, ft/sec2
d = braking distance, ft
t = braking time, sec and the negative sign denotes deceleration

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.

USER TYPE N MEAN (m) 85TH PER (m) FRICTION FACTOR
Bicycle 130 4.8 7.0 0.32
Hand cycle 32 3.9 7.1 0.24
Inline skates 20 8.4 10.8 0.20
Kick scooter 14 4.9 8.9 0.09
Manual wheelchair 9 1.0 1.7 0.23
Power wheelchair 6 2.3 4.6 0.13
Recumbent bicycle 6 3.9 5.3 0.30
Segway 5 2.7 3.1 0.49
Other 24 3.7 6.6 0.28


Table 16. Deceleration rate.

USER TYPE N MEAN (m/sec2) 85TH PER (m/sec2)
Bicycle 130 2.3 3.3
Hand cycle 32 2.3 3.6
Inline skates 20 1.5 2.0
Kick scooter 14 2.4 2.6
Manual wheelchair 8 1.0 1.8
Power wheelchair 6 2.2 3.2
Recumbent bicycle 6 2.9 4.0
Segway 5 3.1 4.7
Other 24 1.9 2.4

NOTE: This table includes only active participants.

The implications of these findings are covered in the "Discussion" section below, under the heading "Sight Distance."

Sweep Width

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).

USER TYPE N MEAN MAX (m) STD DEV MAX (m) 85TH PER MAX (m) 95TH PER MAX (m)
Adult tricycle 4 1.0 0.1 1.0 1.1
Bicycle 501 1.0 0.3 1.2 1.4
Bicycle trailer 6 1.1 0.2 1.2 1.3
Hand cycle 48 0.8 0.1 1.0 1.1
Inline skates 62 1.3 0.2 1.5 1.7
Kick scooter 28 0.9 0.2 1.1 1.2
Manual wheelchair 15 1.1 0.4 1.5 1.8
Pedestrian 63 1.0 0.5 1.3 1.7
Power scooter 1 0.7 NA 0.7 0.7
Power wheelchair 12 0.8 0.2 0.9 1.1
Recumbent bicycle 22 0.9 0.1 1.1 1.1
Segway 8 1.0 0.3 1.1 1.5
Skateboard 11 1.1 0.6 1.2 2.0
Stroller 10 1.0 0.4 1.1 1.6
Tandem 3 0.9 0.0 0.9 0.9
Other 17 1.2 0.3 1.5 1.7

NOTES:

  1. This table includes both active and in situ participants.
  2. AASHTO value for an on-street bike lane = 1.2 m (4 ft)
  3. AASHTO value for a two-way shared use path = 3 m (10 ft)
  4. NA = Not applicable because N=1.

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.

Three-Point Turn

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.

USER TYPE N MEAN (cm) STD DEV (cm) 85TH PER (cm) 95TH PER (cm)
Adult tricycle 2 267 75 265 315
Bicycle 50 287 84 350 371
Hand cycle 30 457 86 541 596
Inline skates 16 160 71 241 262
Kick scooter 12 178 61 210 273
Manual wheelchair 7 113 30 146 155
Power scooter 1 145 NA 145 145
Power wheelchair 8 138 20 152 167
Recumbent bicycle 6 306 36 339 357
Segway 3 98 3 100 101
Stroller 2 286 214 392 422
Other 7 471 221 681 787

NOTES:

  1. This table includes only active participants.
  2. NA = Not applicable because N=1.

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).

Turning Radius

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:

Equation 6: R equals V squared divided by 15 times the results of e divided by 100 plus f.     (6)

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:

Equation 7: R equals V squared divided by 15 times f.     (7)

This formula can be rearranged as

     (8)

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.

USER TYPE N 3.1 m RADIUS 6.1 m RADIUS 9.2 m RADIUS 15.3 m RADIUS 25.4 m RADIUS 27.5 m RADIUS
Bicycle 142 0.61 0.52 0.36 0.26 0.18 0.20
Hand cycle 31 0.19 0.21 0.18 0.15 0.15 0.15
Inline skates 25 0.42 0.46 0.32 0.23 0.11 0.12
Kick scooter 13 0.27 0.21 0.16 0.12 0.07 0.08
Manual wheelchair 7 0.05 0.07 0.05 0.03 0.02 0.04
Motorized wheelchair 4 0.12 0.09 0.07 0.12 0.06 0.07
Recumbent bicycle 6 0.64 0.52 0.37 0.25 0.15 0.16
Segway 4 0.29 0.58 0.29 0.17 0.10 0.09
AASHTO values   0.32 0.30 0.29 0.26 0.24 0.23

NOTE: This table includes only active participants.

Figure 45: Photo. Two tandem riders negotiating a curve at the turning radius station. Two participants on one tandem are negotiating a curve at the turning radius station.
Figure 45. Two tandem riders negotiating a curve at the turning radius station.

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