The majority of our time as pedestrians is spent negotiating level walking conditions, with slipping or tripping an important cause of falls in these circumstances. Whereas intrinsic or individual risk factors appear to be a primary cause of falls in older people (Waller 1978), extrinsic or environmental factors probably initiate most falls among the healthy adult population (Grönqvist 1995a). The latter includes problems with the walking surface (e.g., contamination, irregularities, or obstacles), deficient footwear or inadequate illumination (Grönqvist et al. 2001c; Redfern et al. 2001). Walkway surface contaminants (e.g., water, oil, organic debris, soil, snow) may be involved as contributing factors in as many as 80% of slip-related injury incidents (Grönqvist 1999; Leclercq 1999).