- March 31st, 2022
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For additional local context: Pittsburgh District 8 Councilwoman Erika Strassburger spoke to the 85th percentile rule back in April 2021 as she introduced a Will of Council to urge the state to stop using the rule, citing it as unsafe for our urban streets.
Her plea even made international news via Reuters and other publications in June of last year as talks of President Biden's infrastructure bill were heating up – noting that, "like dozens of other U.S. cities, Pittsburgh is trying to tame automotive traffic and lower the number of pedestrians struck by vehicles, which average 250 per year for the city."
The "85th Percentile Speed" has been discussed more and more frequently in national and regional news – from national tech publications like Wired (summarized in the next article below) to the TribLive article published last April as Pittsburgh officials were seeking authority to set lower speed limits.
To better understand this burgeoning debate about roadway safety in America, below is a summary to help travelers understand the methodologies and approaches historically used by agencies when establishing roadway conditions.
The Manual on Uniform Traffic Control Devices (MUTCD) lists the current speed distribution of free-flowing vehicles as the primary factor to consider when establishing speed limits. It also states that the speed limit should be within 5mph of the "85th percentile speed" – a 1930s city planning rule and engineering principle further championed in the 1960s that opponents argue was antiquated even then.
In a nutshell, the 85th percentile speed is the speed at or below which 85% of drivers will operate with open roads and favorable conditions. The principle is built on the underlying assumption that most drivers will operate their vehicle at speeds they perceive to be safe.
The principle's logic dictates, thereby, that speed limits set above or below the 85th percentile speed will create unsafe conditions due to speed differential as some drivers strictly adhere to the law while others drive the "naturally-induced" speed, and so the 85th percentile has been used nationwide as the first approximation for a road's speed limit, though many municipalities have since moved on to broader, more inclusive approaches that attempt to consider other important factors from like dense urban contexts, foot and bicycle traffic, demographics, crash rates, public input, and more.
If you're interested, jump to pages 12 and 49 of the Federal Highway Administration's (FHWA) 2012 information report, Methods and Practices for Setting Speed Limits for insight on the factors used and adjustments typically made by states and municipalities when setting speed limits. The most common adjustments outlined in the report include:
- Crash data,
- Narrow roadways,
- Narrow shoulders,
- Curves,
- The number of driveways and restricted visibility,
- Rural residential or developed areas, and
- The higher potential for pedestrian and bicycle traffic
According to the FHWA report, setting speed limits based on the 85th percentile was originally based on safety. Specifically, the research at the time had shown that traveling at the 85th percentile speed yields the lowest crash risk for drivers. At the time, it was thought to essentially reflect the collective judgement of the vast majority of drivers selecting a reasonable speed for given traffic and roadway conditions.
Criticisms of the 85th percentile speed principle are that it – again, according to the same FHWA report – assumes motorists are aware of and select the safest speed, and that drivers are generally bad at accounting externalities. Further criticisms suggest that using the 85th percentile method has led to a gradual upward creep in average operating speeds over time. (e.g. Cities across the country have begun instituting Neighborhood Traffic Calming programs – like the City of Pittsburgh's Department of Mobility and Infrastructure's program).
While the MUTCD recommends setting posted speed limits near the 85th percentile speed – and traffic engineers say that agencies are using the 85th percentile speed to set limits – in reality, the speed limit is often set much lower. In many cases, the 50th percentile operating speed is either near or exceeds the posted speed limit, as well, as many agencies deviate from their written guidelines and instead post lower speed limits as a result of political pressures. The report notes, too, though, that it is important to consider that setting speed limits lower than the 85th percentile speed does not encourage compliance with the posted speed limit.
Despite wide-spread use of the method in North America, the report notes that few jurisdictions have quantitative criteria for adjustments to the 85th percentile speed. For example, how much should a speed limit be reduced if there is a high volume of pedestrian traffic on the street?
Another engineering approach for establishing speed limits is the "Road Risk Method" (used in Canada and New Zealand, for example) – which considers the risks associated with the physical design of the road and expected traffic conditions – essentially attempts to set speed limits based on road classifications and then adjust based on relative risks introduced by road design features. Factors like existing speed limit, characters of the surrounding environment, function of the road, roadside development data like the number of houses, shops, schools, etc., the number and nature of side roads, roadway characteristics like medians, number of lanes, street lighting, sidewalks, cycle lanes, parking, setbacks, etc., pedestrian and cycle activity, and more are all considerations.
Disclaimer: The contents of this handbook reflect the views of the authors, who are responsible for the facts and the accuracy of the data published herein. The contents do not necessarily reflect the official view or policies of the Federal Highway Administration (FHWA). This handbook does not constitute a standard, specification, or regulation. It is not intended for construction, bidding, or permit purposes.
