The 85-Percentile Rule is a guide for setting speed limits but one that must be used with a great deal of discretion to prevent creation of unsafe road and streets.
85-Percentile Rule – No, it’s not where you were on the math SATs but a way of setting speed limits on roads and streets. Departments of transportation define the 85-percentile speed as the speed at or below which 85 percent of all vehicles are observed to travel under free-flowing conditions. Traffic and Transportation Engineers use the 85th percentile speed as a guide to set the speed limit at a hopefully safe speed, minimizing crashes and promoting uniform traffic flow. Data are gathered using radar guns or other devices and then analyzed. Engineers take the observed speeds and eliminate the top 15 percent. The indicated speed is then at the 85-percent level (to the nearest 5 mph increment). This procedure – if the set speed is not excessively low – ensures that all drivers will be going at about the same speed with few opportunities for conflict. This, of course, means that 15 percent of the drivers are exceeding what the other 85 percent view as a comfortable and safe speed. So who is correct?
This is fine until the design speed and the posted speed are in conflict.
The Caveat – The catch is the “. . . speed at or below which . . . vehicles are observed to travel under free-flowing conditions.” When design speed – the speed that feels safe or comfortable for most drivers – markedly exceeds the posted speed, we are in trouble. Many people are comfortable driving faster than is safe for them and or comfortable for others especially when there are confounding factors. Those few who adhere to the posted speed will be passed where possible or “tailgated” when not. “Free flowing” also presents a problem if the rule is applied to streets that are, at times, free flowing but at others, not so much.
Right Here, Right Now – A local poster child is Albany’s Washington Ave., an unnumbered State route. It was clearly built – designed – as a 55 mph road. Nonetheless, going outbound from Brevator St. to Rt. 155/New Karner Rd. it is posted at 30 mph (Brevator St. to Fuller Rd.) and 45 mph (after Fuller Rd.) These reduced speed limits are based on the increasing non-highway activity with University at Albany residences and crossing pedestrians, motels, medical and other offices, convenience stores, big-box shopping, social clubs, churches, the dump, and cross roads. There is talk of a massive rebuild of at least the Brevator-Fuller segment but no action to date.
So what went wrong? If we used the 85-Percentile Rule today with the current built up environment, one might guess that the speed thus suggested might well exceed 45 mph and certainly 30 mph. Just drive on it at the speeds posted currently to demonstrate this to yourself. Put another way, the world changed but Washington Ave. did not. We are in the 2000s, and it is in the 1950s. Rest assured that Washington Ave. is not the only such example in our area.
Death Alley – A similar situation is the so-called Cohoes Blvd. or Cohoes’s own “death alley.” Briefly, the State originally built I-787 to blast into the middle of Cohoes with ¾ of the city on the west side and ¼ on the east. The transition from interstate to city street was invisible with expected results. After many years and much pressure, the in-city portion was completely rebuilt featuring raised crosswalks or speed tables, lighting, a chicane, a multiuse side path, enhanced traffic control, a center median, turn lanes, and signage. When asked, a representative stated that NYSDOT would base the redesigned boulevard’s (new) speed limit on the 85-percentile rule. Coming into Cohoes, the posted speed is now 45 mph and then 35 mph. Whether or not drivers follow these limits is for others to judge.
Driver Behavior – One source (https://www.youtube.com/watch?v=NDYQaa3K_BA ) posits that the change from a street to a road challenges drivers to “shift mental gears” from highway mode – which is more or less unconscious, intuitive, and automatic driving behavior – to being focused, observant, engaged, and deliberate. So how does one force fit driver behavior to street conditions?
Appropriate Use – For rural roads and with no countervailing data (e.g., excessive crash data, blind curves, schools, pedestrian and cyclist traffic, no shoulders, many entries/exits) the 85-Percentile Rule is probably a good first step. Note that the term here is “roads” not “streets.”
Inappropriate Use – When we get to a “street” the situation changes. A road is a high-speed connection to get from here to there with a simplified environment with minimal cross streets and driveways, and no accommodation for pedestrians or cyclists.
A street is a complex environment used by people where cars are guests that have to be caused to be on good behavior. A street is not intended for fast motor vehicles as a thoroughfare but a place where people go – the end point of a trip. Setting the posted speed limit based on car travel speeds does not seem like an appropriate application of the 85-percentile rule. In fact, it’s a “cart before the horse” to set the speed limit AFTER the street is built. A better option is to decide on the desired safe speed and THEN spec the design (or alter an existing design).
You cannot just ask for good behavior (e.g., “30 MPH” [please], “Share the Road” [please], “School Zone – 20 MPH” [please] ) you have to design out bad behavior. Drivers do not constantly adjust and re-adjust their speed based on the roadside signage. Instead, they cruise along at a comfortable rate (e.g., the design speed) partially based on what other nearby drivers are doing. The street’s DESIGN is a much more effective speed control than a sign buried with all the rest of the roadside garbage signs that transportation officials believe will offset inappropriate street design. The message from the street’s design is clear with no static; that from the sign (e.g., 30 MPH) has to force its way through a very noisy channel with dismal results.
Back to Our Sample – Washington Ave. becomes a street at roughly Brevator St./Rt. 85 heading toward downtown Albany. That is, the nature of the right of way changes from a road to a street with people, residences, schools, shopping, offices, churches, restaurants, and pedestrian/bicycle traffic. Put another way, Washington Ave. transitions from a road to a “stroad” – a thoroughfare that is neither a road nor a street. This is why, for example, 20 mph speed limits with vehicle speed displays in school zones are a failure – nothing in the road design says “slow down!” As an historical aside, Washington Ave. once had a center median but it was removed in the 1930s – too bad!
They are INTRAstates – In many towns and cities in the USA with street layout dating to the 19th, 18th, and even 17th centuries, streets have become stroads with inappropriate outcomes for all concerned. Even with interstate highways – which have morphed into INTRAstate highways – motor vehicles entering or leaving cities have caused this “stroadization.” Cars come flying off 4-lane, high-speed roads (with a 4-lane, high-speed mentality) and are then faced with narrow, winding, people-filled streets with traffic lights, double-parked cars, snow piles, people trying to cross, squirrels, children going to school, and so on. Without proper STREET design, the result is chaos and, frequently, property damage, injury, or death. Albany, as but one example, is completely ringed and bisected by 4-lane, super highways – 787, 85, 90, NYS Thruway, and 87. Each both feeds traffic into the city but also is a goal for those leaving. “Drive time” has become “all the time.”
High car speeds intrinsically mean greater chances of injury or death for pedestrians or people on bicycles and compression of driver reaction time to the dynamic nature of a street. The solution? There has to be something that tells drivers that (1) the road (or highway) has now ended and (2) the street has now begun. These “somethings” can include narrowing of the road or travel lanes, speed tables or speed humps, vertical objects like trees, center medians, or chicanes.
Conclusion – The 85-percentile rule is a starting point for determining posted speed limits. It is probably appropriate for roads. It is the wrong rule when there are safety and social concerns that supersede the desire of people in cars to “get through here as fast as possible.” Streets are for people. Parks are for people. Intersection crossing are for people. Stores, restaurants, offices, apartments, and houses are for people. A traffic engineer who applies the 85-percentile rule to urban or suburban streets might consider a career shift.
Notes – Strong Towns coined the term “stroad” to explain dangerous, multi-lane thoroughfares that are in every city, town, and suburb. They are what happen when a street – a place where people interact with businesses and residences and where wealth is produced – is combined with a road (a high-speed route between productive places). https://www.strongtowns.org/journal/2018/3/1/whats-a-stroad-and-why-does-it-matter Strong Towns (https://www.strongtowns.org/about ) supports people across the United States and Canada who are advocating for a radically new way of thinking about the way we build our world.
85th Percentile Speed Explained – http://www.mikeontraffic.com/85th-percentile-speed-explained/
Understanding the 85th Percentile Speed – https://www.strongtowns.org/journal/2020/7/24/understanding-the-85th-percentile-speed
The Wrong Way to Set Speed Limits – https://www.youtube.com/watch?v=bglWCuCMSWc