Wednesday, November 16, 2011

Intended speeds vs. design speeds of Chicago's streets

We mentioned in a prior post the “design speed” of a street, which is sometimes different from the intended or posted speed limit. The way a street is configured, either intentionally or not, has the effect of “telling” a driver how fast or slow to proceed. 
What works well for streets in Death Valley National Park doesn't necessarily translate well to Chicago.
(Photo: Bike Walk Lincoln Park)
Elements that make drivers comfortable going fast are: straight sight lines, wide lanes, multiple lanes of traffic going in the same direction, long distances between crosswalks/stop signs/stoplights, and a wide shoulder or lack of obstacles the driver would hit if they suddenly had to make a defensive move like swerving out of their lane. 

Those elements are all appropriate for an interstate highway cutting through the deserts of the southwest, but they aren’t great for dense urban areas like Lincoln Park. Here we’ve got people walking, bicycling, and driving in close proximity to one another, and crossing paths often. We need to have auto traffic moving at speeds that maximizes safety for ALL road users. 
Certain elements of Lincoln Avenue keep vehicles driving at reasonably slow speeds.
(Photo: Bike Walk Lincoln Park)
Some of our Chicago streets have a reasonable, safe posted speed, and a design speed that is approximately the same. Look at the picture of Lincoln Avenue. It has a posted speed of 25 mph, and we feel the design speed is probably about 25-30 mph. It’s kept at relatively slow speeds due to the narrowness of the street, the presence of parked cars (lack of “shoulder” for swerving into), and the presence of a now-well-marked bike lane.

Diversey Avenue at 400 west, as seen on Google Maps Street View.
Similarly, the picture of Diversey Avenue just west of Sheridan above shows a posted speed limit of 30 mph, and we think the design speed is about 30 mph. Again, the fact that it’s two lanes, with parked cars, and numerous signalized crosswalks helps keep drivers’ speed low. (There’s plenty of room for improvement even here, but we’ll not share our suggestions just now.)

However, other streets have a big gap between their intended/posted speed and their design speed. 
The 1900 block of North Clark Street, as seen on Google Maps Street View.
You’ve heard us talk a lot about Clark Street between North Avenue and Armitage (see our ideas for making this stretch of Clark Street safer on this blog post). Its posted speed is hard to find, but we’ll assume it’s 30 mph, which is the default speed limit in the city. However, take a look at the photo above. How fast do you feel comfortable driving on this street? Forty mph? Maybe 45 mph? You’re not alone. But just because it feels safe to drive that fast, doesn’t mean that it IS safe.

Our final example is in River North, and we’re featuring it even though it’s outside Lincoln Park because there’s relevant crash data available. Dearborn Street starting just north of the Chicago River has five wide lanes, with three moving lanes all going in one direction, and two lanes of parking. 

When you design for cars and speed, you get cars and speed. Dearborn Street just south of Kinzie.
(Photo: Bike Walk Lincoln Park)
Look at that picture above. How fast do you feel comfortable driving that stretch of Dearborn? Probably 35 or 40 mph, at the minimum, likely even faster. This is our regular bicycle commuting route home from the Loop (Yeah -- see that skinny little bike lane on the LEFT side of Dearborn in the picture? Don’t get us started...) and we can tell you that cars regularly hit those high speeds as they fly past us. 

The posted speed limit for Dearborn Street in
River North is 20 mph.
(Photo: Bike Walk Lincoln Park)
What do you think the posted speed limit is on this stretch of Dearborn? You’ll be astounded to know it’s 20 mph. So there’s a gap of at least 20 mph between the intended speed and the design speed.

And what’s the effect? According to the most recent data from CDOT, the section of Dearborn Street from Ohio to Huron is THE most dangerous corridor for pedestrians in Chicago’s central business district, especially for fatal and serious injury crashes. The 2011 Pedestrian Crash Analysis says that for every 10,000 pedestrians in that corridor, there were 246.81 crashes, a frighteningly high number (see page 49). Is the poor street configuration and the poor safety record related? We think it’s likely.

We need to help drivers use reasonably low, safe speeds that take into account the safety and well-being of people who are walking and bicycling. All over our city, we need to “right size” our streets’ design speeds, through narrowing lanes, installing safe bike lanes and highly visible crosswalks, adding medians and pedestrian safety islands, and more. 

West Menomonee Street. Design speed: 10 mph. Posted speed: Completely unnecessary.
(Photo: Bike Walk Lincoln Park)
Chicago will soon have new pedestrian and bicycle plans that will outline comprehensive measures to address problems like this. There’s a lot of work to do, and we look forward to seeing what’s in store.


  1. Dearborn is a perfect street for a protected bike lane. One lane can be converted and there is still plenty of capacity for cars.

  2. I agree. Wouldn't it be great to have a protected lane (or other type of very, very safe bike lane) on Dearborn all the way from the Printer's Row to North Avenue? It would open up a world of possibilities for people to ride their bikes to destinations in the South Loop, Loop, River North, Gold Coast and Lincoln Park. -- Michelle Stenzel

  3. I would LOVE to see a protected bike lane on Dearborn from Printers Row to North Ave. It would make this a much safer route for all types of riding compared to the current configuration.