How St. Louis is Allowing MoDOT and the 85th Percentile Rule to Kill Gravois

How St. Louis is Allowing MoDOT and the 85th Percentile Rule to Kill Gravois

Gravois feature

If the city street in front of your home were posted with a 35MPH speed limit would you feel safe? If one of every seven vehicles on the street travelled faster than 47MPH would you be OK letting your children play on the sidewalk or cross the street?

If the traffic on the city street through your neighborhood routinely traveled 46MPH in a 30MPH zone, would you enjoy walking to a restaurant, or feel comfortable parallel parking at a business on the street?

St. Louis is creating dead zones in the city, places with no economic role, streets that decrease adjacent property value and hurt the city’s economy. City leaders are either unable or unwilling to serve the interests of the city and its residents.

We have written extensively about plans for Gravois Avenue, initial acquiescence by city officials and alderman, community engagement, and MoDOT’s willingness to listen and modify its plan. What we didn’t have was the raw information with which MoDOT was basing design decisions. Now, with MoDOT’s Gravois speed study, it’s more obvious that it’s time for the City of St. Louis to re-take ownership of its streets.

Gravois Avenue, among other city streets, is maintained and administered by the Missouri Department of Transportation. This means that the streets inherently serve a statewide purpose. This means that the role of the street is to move as many vehicles as quickly as possible through the City of St. Louis.

The city has been happy with this arrangement as it means that MoDOT money pays for paving, lights, and other infrastructure. However, this “savings” continues to cost the city.

How and why? MoDOT and the City of St. Louis have been planning the repaving and reconfiguration of the almost six-mile long Gravois Avenue from the city limits to I-44 just south of downtown. MoDOT has approached this project as MoDOT approaches projects, applying its standard traffic engineering process.

What this means in a practical sense is that MoDOT conducted a speed study to assess whether there is a need to change any of the existing speed limits along eight segments of Gravois. Five of these segments are currently signed at 30MPH, three at 35MPH.

How does MoDOT determine the correct speed limit? It uses a standard traffic engineering standard known as the 85th percentile. This measurement relies on drivers to set the speed limit. The speed of traffic is measured and it is assumed that the speed 85% of all drivers do not exceed is the correct speed limit. This is often referred to as the “rational” speed limit, meaning rational for drivers who are passing through.

MPH segments graph

There is a practical reason for such as standard. On an open highway it can be assumed that a large majority of drivers will travel at a speed at which they feel is safe, for themselves. Setting the speed limit at that speed is practical. It is assumed that there will always be some drivers (15% or so) who will drive above any posted limit.

Setting a speed limit artificially low creates its own problems. If an open highway is signed at 40MPH a few drivers would travel at the legal speed, but many more would travel at the speed which is comfortable to them. This is why simply changing the speed limit on a road is often not a good way to increase safety.

But what works on an Interstate, or rural highway does not work on an urban street. There are, or should be, considerations in addition to traffic speed and capacity. Does the street design support adjacent property values? Does it support adjacent businesses? Does it support a safe, walkable neighborhood? Does it encourage greater pedestrian activity? Has it increased or decreased tax revenue?

The 85th percentile rule is a terrible way to determine the speed limit of a urban street. Though perhaps this is the argument, whether Gravois through south city is an urban street. It was once. Then everyone bought a car, then two, and drivers demanded that more space be given to them, and that they be given access to wherever, whenever they chose. Cities often acquiesced in the name of progress, or from fear of upsetting drivers.

By signing on to the MoDOT Gravois plan, the City of St. Louis and the aldermen through whose wards Gravois crosses, have failed the city and its residents. The city should initiate a strategic plan now that would reclaim management of its streets, allowing them to serve the city and its businesses and residents first.

What the MoDOT speed study shows is incredible and depressing. Every segment measured (16 total including east and west bound lanes) found 1-in-7 drivers exceeding the speed limit by at least 20%. The worst was the westbound segment from Kingshighway to Morganford where more than 1-in-7 drivers exceeded the posted 30MPH speed limit by 53%. This means they were traveling at 46MPH or faster. This stretch is lined with commercial storefronts and single-family home neighborhoods. This stretch is primarily in the 13th Ward of Alderwoman Beth Murphy. Gravois also crosses the 6th, 7th, 8th, 9th, 12th, 14th, 15th, 20th, 25th Wards.

Grand to Jefferson{Gravois from Jefferson to Grand: signed 35MPH, 85th percentile 43MPH eastbound, 46MPH westbound}

Other speed measurements show more than 1-in-7 eastbound drivers from Hampton to Kingshighway driving 44 in a 30MPH zone, or 47% above the posted limit. The number for eastbound Kingshighway-Morganford and westbound Chippewa-Gustine exceeds 40%, with eight other segments exceeding 30%.

What this means is that drivers are consistently exceeding posted speed limits on Gravois. What this means to MoDOT, using the 85th percentile rule, is that speed limits should not be decreased. What this should mean for the City of St. Louis, aldermen, business owners and residents, is that the street should be designed to slow traffic.

A vehicle traveling at 47MPH, which more than 1-in-7 are doing along Gravois from Jefferson to Russell, is simply not going to come to a stop for a pedestrian in a crosswalk. If 15% of vehicles are moving at 47MPH or faster, no pedestrian, bicyclist, or person in a wheelchair will feel safe.

Morganford to Kingshighway{Gravois from Morganford to Kingshighway: signed 30MPH, 85th percentile 43MPH eastbound, 46MPH westbound (top image show streetview)}

The 85th percentile is a guideline that can accept addition inputs. These, however, are all focused on drivers and not the city, economics, or residents. Factors that can lead to a lower speed limit are the vehicle accident rate, a high number of driveways, consistent presence of pedestrians without a sidewalk, and street parking.

MoDOT’s conclusion from the speed study is that the posted speed limits should remain the same along the entire stretch of Gravois. Again, the department’s process allows no input of the impact of that speed on the homes, businesses, community, and residents. That’s not going to change. What can change is for the city to take ownership, in reality or in action, of its streets.

To highlight another shortcoming of the current planning process, current configurations along Gravois call for unprotected bike lanes next to traffic that routine exceeds 40MPH. This is absolute lunacy. This is something any traffic engineer, let alone a bicycle-pedestrian coordinator (which the City of St. Louis recently hired), should reject.

But the painted bike lanes on Gravois aren’t really made for bikes. They’re simply something to do with left over space after painting traffic, parking, and turn lane widths to a standard size. Nothing other than a protected bike lane, separated from 40MPH traffic, makes sense.

It’s obvious that actually changing the curb-to-curb width would be an expensive endeavor. Not only does MoDOT not want to impede the flow of traffic, it doesn’t have the funds, and won’t directly see the reward, of a drastically altered Gravois. The City of St. Louis should take the lead.

With control of Gravois, the city could create a TIF for the corridor to pay for infrastructure improvements as property values increase and businesses thrive. The city could explore functional bike routes (anyone remember Bike St. Louis?) instead of having painted lanes appear and disappear alongside speeding traffic.

As some neighborhood downtowns (and inner ring suburb downtowns) are making a comeback, the historic commercial districts up and down Gravois suffer. The potential to replicate downtown Maplewood, The Grove, or South Grand exists along several stretches. This is the potential that only the city can act upon.

For all the criticism and energy aimed at MoDOT since the initial plan to close many city streets, it’s the city that bears the responsibility of creating a great place to live and do business. As long as it chooses to allow others, with other priorities and processes bias against urban areas, to plan our city, our places will continue to fail.

[Gravois Avenue project graphic with posted speed limits and recorded 85th percentile speeds]



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