The research literature on access ramps used in transit vehicles is undermined by inconsistent methodologies used across studies, thus providing an inconclusive evidence base for proposed Federal guidelines that would impose a maximum 1:6 slope for all deployment situations. The current study assessed the usability of ramp slope for mobility aid users. Four access ramp slopes were evaluated, with 27 adults representing three populations: manual wheelchair users, power wheelchair users, and people with vision impairment who use a cane or service animal. The dependent variables included five usability measures. The 1:8 and 1:12 slopes were usable and acceptable for most participants. The data indicate that the 1:4 slope is too steep for safe unassisted boarding and disembarking. Many manual wheelchair users lacked the strength needed for unassisted ascent. Power wheelchair users and people with vision impairment expressed safety concerns about descent of steeper slopes. Conclusive interpretations should be cautiously drawn because the sample size was relatively small and did not include users of scooters or ambulation aids.