Designing City Streets to Suit 47 MPH Drivers Is a Recipe for Failure

On a road like St. Louis' Gravois Avenue, applying the highway standard 85th percentile rule to establish a speed limit is dangerous and damaging, says Alex Ihnen. Photo: NextSTL
On a street like St. Louis’ Gravois Avenue, applying the “85th percentile rule” will kill street life. Photo: NextSTL

Gravois Avenue is an important commercial street in St. Louis that also happens to be designated a state highway. It’s currently slated for a redesign, providing a huge opportunity to make the street work better for walking and biking.

But unfortunately the highway-like mentality of state transportation planners persists. Alex Ihnen at NextSTL reports that Missouri DOT is using highway design strategies, and the city of St. Louis is letting them get away with it. Instead of redesigning the street to work for city residents, Ihnen writes, MoDOT is looking at how fast people already drive, and making decisions to accommodate drivers who travel faster than 85 percent of other drivers:

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?

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…

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.

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.

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.

Ihnen reports that the speed study found that many drivers were speeding, traveling 45 miles per hour or higher. But this is no reason to design the street to accommodate their behavior:

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.

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.

Elsewhere on the Network today: Bike Portland surveys who is supporting a local gas tax hike and why. Twin Cities Sidewalks rails against “death roads,” four-lane divided highways in urban areas. And FABB Blog rounds up new research showing an overwhelming majority of American mayors are supportive of bike infrastructure.


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