Abstract

The visual accessibility of a space refers to the effectiveness with which vision can be used to travel safely through the space. For people with low vision, the detection of steps and ramps is an important component of visual accessibility. We used ramps and steps as visual targets to examine the interacting effects of lighting, object geometry, contrast, viewing distance, and spatial resolution. Wooden staging was used to construct a sidewalk with transitions to ramps or steps. Fortyeight normally sighted subjects viewed the sidewalk monocularly through acuity-reducing goggles and made recognition judgments about the presence of the ramps or steps. The effects of variation in lighting were milder than expected. Performance declined for the largest viewing distance but exhibited a surprising reversal for nearer viewing. Of relevance to pedestrian safety, the step up was more visible than the step down. We developed a probabilistic cue model to explain the pattern of target confusions. Cues determined by discontinuities in the edge contours of the sidewalk at the transition to the targets were vulnerable to changes in viewing conditions. Cues associated with the height in the picture plane of the targets were more robust. Keywords: visual accessibility, low vision, mobility, visual acuity, visual contrast, visual recognition, steps, ramps Citation: Legge, G. E., Yu, D., Kallie, C. S., Bochsler, T. M., & Gage, R. (2010). Visual accessibility of ramps and steps. Journal of Vision, 10(11):8, 1-19, http://www.journalofvision.org/content/10/11/8, doi:10.1167/10.11.8. Introduction The visual accessibility of a space refers to the effectiveness with which vision can be used to travel safely through the space and to pursue the intended activities in the space. Our long-term goal is to provide tools to enable the design of safe environments for the mobility of low-vision individuals and to enhance safety for others, including older people with normal vision, who may need to operate under low luminance, glare, and other visually challenging conditions. A long-term goal of our research is the development of a computer-based design tool in which complex, real-world environments (such as a hotel lobby, large classroom, or hospital reception area) could be simulated with sufficient accuracy to predict the visibility of key landmarks or obstacles under a variety of natural and artificial lighting conditions. This paper reports on our study of the detection and recognition of single steps (up or down) and ramps in a simple, indoor environment. Subjects had normal vision but made judgments under conditions of blur simulating reduced acuity. Our goal was to explore the interacting effects of lighting direction, target/background contrast, viewing distance, and blur. We conducted our psychophysical measurements in a real space rather than simulating stimuli on a computer screen or in a virtual environment to ensure that we captured the complexity of the real world. We reasoned that it is important to understand the visual cues and other factors determining visibility of groundplane irregularities in a simple real-world space before attempting to generalize the analysis to a wider range of realistic environments and the performance of visually impaired subjects. It is often difficult for a normally sighted person to judge when features, such as steps, are hard to see because of the complex interactions between lighting, the geometry of the feature, and its surface material. A feature that is easy to see from one viewpoint under diffuse lighting might "disappear" in directional lighting, or one that is easy to see under directional lighting might not be seen under diffuse lighting. Brabyn, Schneck, Haegerstrom-Portnoy, and Lott (2004) presented some compelling photos illustrating the effects of mild contrast reduction and glare on face images and everyday sidewalk and driving scenes. Their goal was to simulate the milder visual impairments of the normal aging eye rather than the more severe loss of spatial resolution typical of low vision. They pointed out that it is difficult to imagine or predict the nature of the substantial functional deficits associated with these forms of mild visual impairment. Arditi and Brabyn With the exception of Goodrich and Ludt The importance of the visual accessibility of environments, particularly ramps and steps, is further emphasized by the large literature showing associations between vision and falls or other accidents in the elderly. For instance, there are associations between reductions in binocularity, contrast sensitivity, acuity, and visual field size and the occurrence of falls and hip fractures in the elderly Our test bed was a sidewalk, built in an indoor classroom Through introspection, we identified a set of cues useful for distinguishing among the five targets. These cues are illustrated in Step UpVthe luminance contrast marking the transition from sidewalk to riser and the kink in the boundary contour of the sidewalk. Panel B shows a cue for Step DownVthe L-junction in the boundary contour of the sidewalk. Panel C shows a cue for Ramp UpVthe bend in the bounding contour associated with the transition from sidewalk to ramp. A bend in the opposite direction is a cue for Ramp Down. Another cue for distinguishing among the targets is the height in the picture plane of the horizontal bounding contour between the far edge of the target and the wall behind it. There are three values for this picture-height cue: high for Step Up and Ramp Up, low for Step Down and Ramp Down, and intermediate for the Flat target. The visibility of these cues depends on the contrast of the boundary contours of our five targets and in some cases (such as the L-junction for Step Down) on the angular subtense of a local geometrical feature. Boundary contrast is affected by lighting direction and the contrast between the targets and their backgrounds. Visibility of Journal of Vision Methods Stimuli All experiments were conducted in a large windowless 33.25 Â 18.58 ft (10.13 Â 5.66 m) classroom in the basement of Elliott Hall on the campus of the University of Minnesota. Hardboard deck portable stage risers were used to construct a sidewalk, 4 ft wide by 24.5 ft long (1.3 Â 7.5 m), elevated 16 in. (0.4 m) above the floor. The sidewalk was painted with Valspar satin light gray porch and floor enamel. One of five possible targets formed a continuation of the sidewalk at its south end. Step Up, Step Down, Ramp Up, Ramp Down, and the Flat continuation of the sidewalk. The five targets were formed by arrangements of a 4 Â 8 ft (1.2 Â 4.3 m) Â 2 in. thick rectangular panel of expanded polystyrene (EPS) and covered with the same gray paint. An additional small block of painted EPS was glued on the near (viewable) end of the EPS panel to create the front riser seen in the Step Up condition. The polystyrene surface of the target panel and the wooden surface of the sidewalk could be distinguished visually by fine texture differences with normal vision but were indistinguishable under blur. The five targets were configurations of the polystyrene panel produced by raising (or lowering) one or both of its ends by 7 in. (18 cm) above or below the level of the wooden sidewalk using motorized scissor jacks. To produce a nearly seamless transition between the end of the sidewalk and the target panel, a wedge-shaped block of EPS was fixed to the end of the sidewalk, and a quarter inch sheet of hardboard material was laid over the viewing end of the sidewalk and the extending EPS wedge. Precise alignment of the stimuli was facilitated by two remotecontrolled laser diodes mounted on the target panel, and laser targets were placed several feet away from the lasers. The experimenter changed targets between trials in about 20 sec by operating the jacks with a controller. The classroom floor, far wall, and right-hand wall formed the visual background for the targets. They were paneled with sections of polystyrene, painted gray to match the targets and sidewalk, or painted black (Valspar interior satin dark kettle black acrylic latex) to form a high-contrast boundary with the targets. There were three lighting arrangementsVOverhead, Near Window, and Far Window. Overhead lighting was produced by the room's four rows of three 2 Â 4 ft luminaries (recessed acrylic prismatic 4 lamp SP41 fluorescent). Overhead lighting produced a luminance of approximately 68 cd/m 2 on the gray sidewalk and target panel. There were two "window" conditions, in which the room lighting was turned off and an artificial window was placed at the near and far locations indicated in