Statement of Purpose Congestion pricing policies (CPPs), a travel demand management strategy for alleviating roadway congestion, are a frequently considered and debated strategy in large urban areas (e.g., Manhattan). CPPs can affect many aspects of peoples’ lives with a range of potential impacts on travel efficiency, physical activity, air quality, transportation equity, and safety, among others. The purpose of this study was twofold. We sought to 1) describe the scope of and patterns in the CPP evidence base over time and identify research gaps and 2) conduct a detailed review of safety-related impacts of these policies.
Methods/Approach We conducted two structured reviews of four large citation and research databases from inception through early 2021. We used bibliometric analysis methods, including semi-supervised and machine learning techniques, to screen literature and innovative, network data visualization methods to assess research trends and gaps.
Results The most frequent research themes in the CPP literature (n=2,333) included discussion around policy implementation characteristics and details, consideration of transportation modeling methods and approaches, public perception/acceptability of CPPs, and specific consideration of CPP impacts (e.g., traveler choices, emissions). Key gaps included research on specific road user types (particularly vulnerable road users), equity considerations, and safety impacts. The small group of studies that focused on safety impacts (n= 18) often examined overall crashes and injury crashes, generally finding decreases. However, the subset of studies (n=6) that examined impacts on vulnerable road users generally cited short-term increases in crashes and injuries followed by long-term decreases, potentially due to travel mode shifts.
Conclusion We identified several critical research gaps in the CPP research base. The notable gap in safety research, and the broad range of CPP types and implementation contexts, indicates that more research on CPP impacts on road user safety (for all road users) is warranted.
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