Measuring the LSD effect: 36 percent improvement

The curve at Lake Shore Drive and Oak Street in Chicago is a favorite nudge. The tight turn makes it one of the city’s most dangerous curves. To try and limit wrecks, in September 2006 the city painted a series of white lines perpendicular to traveling cars. The lines get progressively narrower as drivers approach the sharpest point of the curve, giving them the illusion of speeding up, and nudging them to tap their brakes.

Exactly how effective have these lines been in preventing crashes? Until now, only anecdotal accounts have been available. What about a little hard data? According to an analysis conducted by city traffic engineers, there were 36 percent fewer crashes in the six months after the lines were painted compared to the same 6-month period the year before (September 2006 – March 2007 and September 2005 – March 2006).

To see if it could make the road even safer, the city installed a series of overhead flashing beacons, yellow and black chevron alignment signs, and warning signs posting the reduced advisory speed limit. Again, accidents fell – 47 percent over a 6-month period (March 2007 – August 2007 and March 2006 – August 2006). Keep in mind that the post-six-month period effect included both the signs and the lines.

How much more these calming signals will affect driving is unknown, but the city considers the numbers a sign of success. A drive between the North and South Sides is now safer and quicker for everyone.

Hat tip: Chicago Department of Transportation for providing its data.

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