This report describes the results of several on-going analyses: (1) Analysis of ambient data measured under high-wind conditions; (2) Quantification of the differences between in situ observer logging versus offline review of digital recordings; and (3) Computation of natural ambient and existing ambient without air tours.

The National Highway Traffic Safety Administration recognizes that quieter cars such as hybrid-electric vehicles in low-speed operation using their electric motors, may introduce a safety issue for pedestrians who are blind. This study documents the overall sound levels and general spectral content for a selection of hybrid-electric and internal combustion vehicles in different operating conditions, evaluates vehicle detectability for two ambient sound levels, and considers countermeasure concepts that are categorized as vehicle-based, infrastructure-based, and systems requiring vehicle-pedestrian communications. Overall sound levels for the hybrid-electric vehicles tested are lower at low speeds than for the internal combustion engine vehicles tested. There were significant differences in human subjects' response time depending on whether electric or internal combustion propulsion was used at both the lower and higher levels of ambient sound. Candidate countermeasures are discussed in terms of types of information provided (direction, vehicle speed, and rate of speed change, etc.); useful range of detection of vehicles by pedestrians, warning time, user acceptability, and barriers to implementation. This study provides baseline data on the acoustic characteristics and auditory detectability of vehicles; however, the results cannot be generalized to more complex environments, as for example when multiple target vehicles are present.