Space Requirements for Wheeled Mobility Devices in Public Transportation: An Analysis of Clear Floor Space Requirements

Abstract

Recent research on the anthropometry of wheeled mobility devices and their users (n=369) indicates that the current dimensions for ‘clear floor area’ prescribed in U.S. accessibility standards for transportation are inadequate for accommodating many users of wheeled mobility devices, especially those that use power chairs and scooters. The current report presents anthropometry data for determining the dimensions of clear floor area based on occupied device length and width to achieve a specified level of physical accommodation. The implications of the findings and the need for revising guidelines for accessible public transportation systems are discussed. It is important that the transportation industry as well as mobility device manufacturers, vendors and prescribers understand the limitations of current standards and becomes involved in the dialogue about how to address the need for improving them.

INTRODUCTION

Designing for physical accommodation constitutes one major aspect of accessibility in built environments and public transportation systems. Currently, accessibility standards and codes are used throughout the U.S. and in other countries to implement laws that mandate accessibility to buildings (e.g. public restrooms, bus stops, transit facilities, etc.) and transportation (e.g. buses, trains, etc.). The standards for accessibility are based on research in anthropometry, a scientific discipline involving the measurement of body sizes and physical abilities of different user groups, such as ambulatory adults, elderly, and wheeled mobility device users. Although being a relatively small minority estimated at about 3.2 million adults in the U.S. population (1), users of wheeled mobility devices constitute a fast growing segment of the population (2). They also present a unique set of challenges for physical accessibility in public transportation because of concerns over large spatial requirements for floor area and maneuvering.

The Americans with Disabilities Act Accessibility Guidelines (ADAAG) for Transportation Vehicles (3) is the key document used for design of accessible facilities and transportation systems. One of the primary features of the ADAAG is the specification of a ‘clear floor area’ for wheelchairs, a space 760 mm (30 in.) wide by 1220 mm (48 in.) in length. The ‘clear floor area’ is used to determine the size of the area planned for wheelchair securement, the width of doorways and minimum interior clearances for maneuvering a wheeled mobility device, and the space needed to accommodate people in wheeled mobility devices in bus shelters, bus stop pads and in terminals - hence, a critical component for ensuring physical accessibility.

At the time the research that served as the basis for the ADAAG was completed in the late 1970’s, most wheelchairs were predominantly manual, and quite similar in size and style (4). Today, the range of wheeled mobility devices available is much more diverse. Devices are now available in many seat widths; power chairs and scooters are much more prevalent than before. Manual wheelchairs are more diverse due to the availability of features that accommodate individual needs, e.g. different seat widths, lightweight folding chairs, rigid frames, etc. A range of accessories and customizable features are also available such as seating and back cushions, “tilt in space”, adjustable arm-rests, and foot-rest extensions. The growing availability and use of scooters and power chairs is expected to further change the mobility device user demographics in the coming years (5). Wheeled mobility users are more diverse as well. Power chairs have allowed more severely disabled people to become mobile in the community. Scooters are being adopted by people who can walk and stand but not for long distances. People are larger due to the obesity epidemic resulting in a growth in bariatric wheelchairs.

The IDeA Center has been developing an extensive anthropometry database of wheeled mobility devices in the U.S. with the intention of improving accessibility to buildings and facilities, and more recently to improve accessibility in public transportation. The IDeA Center research was initiated to understand the spatial implications of contemporary wheeled mobility technology and user populations, and to provide an evidence base to evaluate existing accessibility standards (6, 7).

The objective of this paper is to evaluate the level of accommodation achieved for the IDeA Center study sample of mobility device users based on the current requirements for minimum ‘clear floor area’ specified in the ADAAG. Second, an approach to computing the minimum required dimensions of clear floor space to attain a desired level of physical accommodation based on occupied device length and width for the measured sample is described. The findings from this analysis are used to provide recommendations for revising the existing guidelines for minimum ‘clear floor area’ dimensions. The implications of inadequate floor space, the need for increasing the ‘clear floor space’ dimensions for wheeled mobility devices on buses and public transportation systems, and some of the inherent technological challenges to achieving this are highlighted.

METHODS

A diverse group of adults who use manual chairs, power chairs and scooters were recruited in Buffalo and Pittsburgh. The sample population had disabilities including arthritic disorders, central nervous system (CNS) disorders, spinal cord injuries, and amputations. Individuals were recruited through many sources, including a local independent living center, a United Cerebral Palsy Association, a geriatric day care center, and local hospitals, including Veterans Affairs Medical Centers in Buffalo and Pittsburgh. In addition, advertisements were posted in local newspapers and flyers placed in local organizations and stores. Only those who relied on a wheeled mobility device for their primary means of mobility were eligible for participation. A deliberate attempt was made to select a diverse group of users, rather than just individuals who possessed a specific set of physical capabilities so that the results obtained could be extended to the broader wheeled mobility device user population in the U.S.

The measurement protocol included the collection of demographic information, mobility device specifications, structural anthropometric measurements and functional anthropometric information from each participant. Other data that were collected but not described in this paper included functional reach capabilities in an object transfer task, device maneuvering abilities, and grip size and strength capabilities.

The protocol for recording structural anthropometric measurements used in this study made use of an electromechanical probe (8). The probe comprised of an articulating arm with six degrees of freedom that had a measurement precision of 0.3 mm and was used to record three dimensional locations of body and wheelchair landmarks with respect to one another and to reference planes (e.g. floor, chair seat, seat back plane, etc). Over 130 standardized body and mobility device landmark locations from each participant were digitized by means of this probe. Three dimensional coordinates of the landmarks were then used to derive estimates of widths, heights and depths of key mobility device characteristics and body dimensions. This method of obtaining anthropometric dimensions provides reliable measurements but differ from those obtained with more traditional anthropometric measuring devices (9).

Three of the computed dimensions included the occupied length, occupied width and occupied floor area of the occupant and their mobility device, with individuals holding a comfortable or typical seated posture. The occupied length for each individual was the horizontal distance from the extreme rear-most and forward-most point of the combined occupant or mobility device. For manual and power chairs, the forward-most point typically was the toe or leading edge of the footrest, while the rear-most point consisted of the trailing edge of the wheel, back pack or edge of the power base in the rear for power chairs. For scooters, the forward-most point was usually a basket that hung off the steering post or part of the front wheel housing, while the rear-most point was usually the rear edge of the housing. The occupied width of the occupant and device was the horizontal distance between the extreme lateral right-most and left-most points on the body or mobility device with the participant seated in a comfortable, relaxed posture. For manual wheelchairs, this could be the outside or lateral edge of the wheel push-rims, the elbow or the hand. For power chairs, the lateral-most landmarks were typically the outside edge of the device controller (e.g. joystick) or arm rests, an elbow or forearm extending over the side of the chair. For scooters, these landmarks could be the outside edge of the device housing or armrests, the participants’ elbows, arms or legs, extending beyond the outside edges of the device housing. Further, a composite measure of occupied floor area for each participant, defined as the product of the occupied length and occupied width, was used to represent the estimated rectangular floor space required for each occupant-mobility device in the study sample.

DATA ANALYSIS

Descriptive analyses were performed to evaluate the distributional characteristics of each of the dimensions. The analysis results were stratified by mobility device type, namely, manual chairs, power chairs and scooters. The mean, standard deviation, range, 5^th and 95^th percentile values of the distribution for each of the three dimensions were also calculated. All data were analyzed using SPSS version 16.0 (10).

A scatter-plot depicting the bivariate distribution between occupied width and occupied lengths was constructed to show the relationship between these two variables. The strength (magnitude and direction) of the association between these two variables was assessed with the Pearson’s correlation coefficient (R), which lies within the range of −1 to 1. The Pearson’s correlation coefficient between occupied width and occupied length was calculated for each of the three types of mobility devices. Lastly, a percentile plot was constructed showing the cumulative percentage of study participants who recorded an occupied floor area of value equal to or less than those measured in the study sample.

RESULTS

The sample comprised of 213 (58%) males and 156 (42%) females. Among males, 55% were manual chair users, 38% power chair users, and 7% scooter users. On the other hand, 50% of the female mobility device users used a manual chair, 42% used a power chair and 8% used a scooter. The mean (standard deviation) age of the sample was 52.4 (17.1) with a range of 18-94 years of age. Regarding self-reported medical conditions that led to dependence on mobility devices, CNS disorders (e.g. multiple sclerosis, cerebral palsy, etc.) were the most frequently occurring (34%), followed by spinal cord injuries (26%), orthopedic injuries/deformities (13%) and cerebral vascular diseases such as stroke (10%). Instances of medical conditions such as amputations (3%), traumatic brain injuries (2%), or respiratory and cardiovascular diseases (2%), were less frequent, with the remaining cases listed under ‘Other’ (10%). It should be noted that these proportions are very different from what might be expected in the adult U.S. population of wheelchair users. For example, there are an estimated 84% manual chair users, 8% power chair users, and about 8% scooter users in the adult U.S. population of wheeled mobility device users (11). Overall, the current study sample had a much larger proportion of power wheelchair users and more severe physical limitations in comparison to the U.S. population. However, power chair users were intentionally oversampled to obtain a better understanding of the functional abilities of this user group, which typically has more severe physical limitations and is hence more sensitive to design restrictions. Additional data collection efforts to increase the size and diversity of the study sample are still ongoing.

Univariate analysis of occupied width, occupied length and occupied floor area

The descriptive statistics for occupied width, occupied length, and occupied floor area stratified by mobility device type are summarized in Table 1.

TABLE 1.

Summary Statistics For Occupied Width (mm), Occupied Length (mm) And Occupied Area (m²) For Manual Chairs, Power Chairs And Scooters

Manual chairs (n=195)
Dimension	Mean	SD	Min	Max	Percentile
					5th>	50th	95th
Occupied Width (mm)	681	59	508	932	595	675	782
Occupied Length (mm)	1141	131	743	1626	919	1148	1342
Occupied Area (m2)	0.78	0.12	0.44	1.25	0.57	0.78	0.99
Power chairs (n=146)
Dimension	Mean	SD	Min	Max	Percentile
					5th	50th	95th
Occupied Width (mm)	707	75	574	1008	605	694	828
Occupied Length (mm)	1196	123	831	1487	981	1184	1399
Occupied Area (m2)	0.85	0.14	0.51	1.27	0.63	0.84	1.11
Scooters (n=28)
Dimension	Mean	SD	Min	Max	Percentile
					5th	50th	95th
Occupied Width (mm)	650	89	540	857	540	617	840
Occupied Length (mm)	1212	107	1044	1439	1044	1203	1435
Occupied Area (m2)	0.79	0.16	0.56	1.18	0.58	0.78	1.14

Scatter-plot of occupied width vs. occupied length

Figure 1 shows a scatter plot of occupied width vs. occupied length. Each of the points on the plot depicts an individual case in the sample (n=369), and are categorized by the type of mobility device. The minimum requirements for ‘clear floor area’ prescribed by the ADAAG (760 mm width x 1220 mm length) is overlaid on the figure to provide a visual representation of the proportion of individuals and devices that have both length and width within the current minimum ‘clear floor area’ standard (Quadrant 1) vs. the proportion of individuals that exceed the minimum ‘clear floor area’ standard either due to occupied length (Quadrant 2), occupied width (Quadrant 3) or exceed on account of both, occupied width and length (Quadrant 4). Table 2 provides the frequency count and percentages in parenthesis (percentages are computed based on the corresponding mobility device category) of the number of cases in each of the four quadrants depicted in Figure 1.

FIGURE 1.

Scatter-plot of occupied width vs. occupied length, overlaid with the ADAAG requirement for minimum ‘clear floor area’ of size 760 mm x 1220 mm (30 in. x 48 in.).

TABLE 2.

Number And Percentage Of Individuals And Mobility Devices With Lengths And Widths Within (Quadrant 1) And That Exceed (Quadrants 2, 3, 4) Current Standards Based On The Minimum Requirements For ‘Clear Floor Area’ Of Size 760 mm X 1220 mm (30 in. X 48 in.) Prescribed By ADAAG

Mobility Device Type	Quadrant				Total
	1 (Included) L<1220 & W<760	2 (Exceed length) L>=1220 & W<760	3 (Exceed width) L<1220 & W>=760	4 (Exceed length & width) L>=1220 & W>=760
Manual	136 (70%)	42 (22%)	11 (6%)	6 (3%)	195 (100%)
Power	71 (49%)	42 (29%)	13 (9%)	20 (14%)	146 (100%)
Scooter	15 (54%)	10 (36%)	1 (4%)	2 (7%)	28 (100%)
Total	222 (60%)	94 (25%)	25 (7%)	28 (8%)	369 (100%)

The analysis of occupied area (i.e. occupied width x occupied length) for mobility device users in the study sample indicated that only about 70% of the manual chair users, 49% of power chair users and 54% of the scooter users in our study sample had dimensions that were within the standard for ‘clear floor area’ of size 760 mm x 1220 mm (Quadrant 1). Increasing the proportion of individuals accommodated would require providing space beyond the existing minimum requirements. Additionally, the data suggests that a greater proportion of individuals were excluded due to inadequate clear floor length (Quadrant 2) as compared to clear floor width (Quadrant 3), or both, inadequate length and width (Quadrant 4).

Pearson’s correlation coefficients between occupied width and occupied length were 0.18 for manual chairs, 0.31 for power chairs, and 0.49 for scooters. The results suggest that large widths do not imply large lengths, and small widths do not imply small lengths; an association that would have otherwise been indicated by highly positive correlation coefficients (e.g., 0.7 to 0.99). Rather, the weak correlations revealed by this data set suggests that neither occupied width or occupied length can necessarily be used to reliably predict the other.

Percentile plot for clear floor area

Percentile analysis of occupied floor area, as shown by Hunter (12), provides a more useful starting point for determining suitable dimensions for ‘clear floor area’. Figure 2 shows the percentile plot for floor area, and plots the occupied floor area for each participant vs. the corresponding percentile value. Considering a minimum accommodation level of 95% (i.e. the values for occupied floor area that encompasses at least 95% of each of the three sub-groups of manual chair, power chair and scooter users), the corresponding occupied floor areas observed in the study sample can be determined (denoted by the three vertical dotted lines in Figure 2).

FIGURE 2.

Percentile plot for occupied floor area, stratified by mobility device type.

From the list of all the occupied widths and occupied lengths that produced an area less than or equal to this 95^th percentile floor area, the largest occupied length and largest occupied width across the sub-groups were found to be 820 mm x 1420 mm (32 in. x 56 in.) for manual chair users, 850 mm x 1480 mm (33.5 in. x 58 in.) for power chair users, and 860 mm x 1440 mm (34 in. x 57 in.) for scooter users. Dimensions for clear floor area determined by these dimensions would ensure that at least 95% of the corresponding study sub-sample would have occupied width and occupied length values less than or equal to these critical dimension values. Specifically, about 97% of manual chair users, 96% of power chair users, and 100% of scooter users from the current study sample could be expected to be accommodated. Note that the high level of accommodation among scooter users was influenced by the relatively small number of scooter users in the study sample (n=28). Further, considering just the maximum values from among these three sets of dimensions for manual chair, power chair, and scooters, a clear floor space of 860 mm x 1480 mm (34 in. x 58 in.) would ensure that at least 95% of any of these three mobility device types would be accommodated.

DISCUSSION

Contemporary wheeled mobility technology can support greater independence, better utilization of public transportation and increased social participation of individuals with disabilities into mainstream society (e.g. employment, shopping/doctor visits, recreational travel). However, the full potential of this technology cannot be realized unless device size and shape, and the size of occupants are accommodated by transportation systems to insure access to safe, comfortable and timely travel. The findings of the research currently underway are expected to guide the revision of standards to ensure this accommodation, insofar as technically and economically feasible. This data can help policy makers and accessibility standards developers evaluate and revise current policies.

It should be noted that measurements in this study were taken with participants seated in a comfortable posture that they could maintain for the duration of the measurement process; typically around 15-20 minutes. Participants often chose to rest their arms on the armrest and/or extend the footrests and legrests to support the lower extremities. This data does not take into account the ability of some individuals to move their limbs inboard of their devices, swing legrests and footrests out of the way, or adjust back-packs and bags which may extend beyond the boundaries of their devices. It is also worth noting, however, that traveling on public transportation is an activity that requires remaining relatively stationary for extended periods of time. While these measurement conditions and postures may tend to overestimate the occupied width and occupied length dimensions for very short time durations, they do reflect postures that wheelchair occupants’ consider comfortable for times that are commensurate with the vast majority of transit trips. Further, the samples were drawn largely from cold weather cities which may introduce some bias toward larger and more durable equipment, but data was collected all year round in an attempt to minimize any bias in recruitment associated with the seasons. Efforts to increase the sample size and diversity through data collection in other cities are ongoing.

The analyses presented in this paper are based on the assumptions of a rectangular area required for static positioning of a wheeled mobility device. The findings suggest that the current minimum requirements of a 760 mm (30 in.) wide by 1220 mm (48 in.) length of ‘clear floor area’ are clearly inadequate for a sizeable proportion of wheeled mobility device users measured in this study, especially those with the most serious disabilities, namely, power chair users. Wheeled mobility anthropometry dimensions from this study have also been compared with existing accessibility standards and anthropometry research findings from other countries such as Australia, Canada and U.K. (6). These international comparisons also suggest a trend toward an increase in occupied floor area among mobility devices, and the inability of accessibility standards to reflect the space requirements of wheeled mobility device users in the built environment. One approach to increasing the level of accommodation would be to provide a larger clear floor area than the existing standards. The findings described here are intended to provide guidance for modifying standards, particularly in applications where the amount of space available in some vehicles is very limited. A large number of individuals in the study were excluded by the minimum clear floor area requirements due to larger occupied lengths (Quadrant 2 in Figure 1), suggesting that the greatest improvements in accommodation could be first achieved by increasing the length dimension of the ‘clear floor area’. Also, space requirements for maneuvering into and out of these spaces were not considered in the current analysis and might suggest the need for additional clearances.

It is important to note that there are technological constraints that exist which may prevent a space increase in public transit environments. The findings need to be the subject of considerable dialogue in the transportation community to determine what level of accommodation is realistic and practical with regard to contemporary vehicle technology, e.g. distance between wheel wells in low floor buses. Involving wheeled mobility aid manufacturers, mobility device vendors and prescribers, representatives from transit agencies and organizations serving individuals with disabilities in this dialogue is also very critical to arriving at acceptable and easily implementable solutions (13).

The results presented here are not only useful for improving regulations and policies but also to identify design goals for future transportation and wheeled mobility technology. For example, the data provide a goal for design of new suspension systems for low floor buses that could increase the available space between wheel-wells, for new securement systems that might allow additional available room in the securement area, or alternative vehicle interior layouts that offer better access and circulation spaces. Also important, in this regard, are innovations in mobility device design that improve device maneuverability making them more adaptable to constrained environments such as on buses, or technologies that increase the comfort and postural support for the occupant while possibly reducing the occupied floor area.

The IDeA Center is currently using the data to develop graphical and interactive design tools that can be used by architects and designers to visualize and understand the impact of physical design features on accommodation levels. Designing the interior of buses and other transit vehicles for accessibility is a multi-dimensional design problem. In some instances, it can require simultaneous consideration for additional variables such as shoulder widths and heights, knee and toe clearance widths and heights and functional reach capability to name a few. Future research activities will explore the development of multivariate analyses tools that can factor in additional variables besides width and length of the occupant and device. Additionally, the relationship between occupied device dimensions and actual performance in dynamic activities such as boarding/disembarking and maneuvering within the bus are currently being investigated with laboratory-based experiments using a full-scale mock-up of a bus with reconfigurable interiors.

CONCLUSION

This research shows the importance of linking occupied width and length of wheeled mobility devices when establishing suitable dimensions for minimum ‘clear floor area’ in accessibility standards, including those used in the transportation sector. It also demonstrates the utility of the assembled anthropometry database in helping standards developers and designers improve accessibility and safety for travelers using wheeled mobility devices on buses, other public transportation vehicles and related built environments.

The results suggest that the current standards are not adequate to accommodate a large proportion of contemporary wheeled mobility users. Three design strategies could address this finding in practice. The standards for minimum clear floor area could be increased. This would have limited, if any, implications for new construction in the built environment but could have major implications for design of some vehicle types, especially low floor buses where the space between wheel wells is very limited. The use of a four-passenger longitudinal flip-up seat in place of a three—passenger longitudinal flip-up seat in one wheelchair securement area would provide enough space for larger scooters and wheelchairs, reducing overall seating capacity by only one person. Finally, location of boarding ramps at entries in the middle of low floor vehicles instead of front door access could increase the maneuvering space available by avoiding the need for passage between wheel wells. This last strategy would require using a fare collection method that did not depend on face to face driver interaction for passengers entering the accessible entry. It also may have, at present, a limited application to boarding from raised platforms because in most low floor buses, the kneeling feature, which enables a lower ramp slope, is only installed on the front of the bus. These strategies above are not mutually exclusive. Together, they provide policy makers with some options that could lead to a reasonable solution with minimum cost and operational impact. The alternative is to enact operating policies that restrict access on mass transport vehicles to devices that have footprints that fall within specified limits. However, our findings indicate that it is not always the device that results in larger space needs. Often, postural requirements such as extended foot-rests or greater seat-back inclination angles due to medical reasons as well as large body size contribute to this effect (7). Such a policy would also create an additional burden for paratransit operations. Moreover, a policy based on size could be considered discriminatory and require drivers to make difficult judgments in the field.