Chicago tricked drivers' own sense of speed into slowing them down at a dangerous curve

the problem

A dangerous curve kept causing crashes no matter how many speed-limit signs the city posted

background

what everyone would do

The standard response to a dangerous curve causing repeated crashes is more explicit warning: lower the posted speed limit, add more prominent signage, install additional warning signs — all of which require a driver to consciously notice, read, and process the instruction in the brief window before they need to act on it.

what they saw

Chicago's engineers saw that drivers weren't crashing because they lacked information about the speed limit — they were crashing because the information they were actually acting on, their own felt sense of how fast they were going, didn't match reality, and no amount of additional signage changes what a driver's senses tell them moment to moment. Rather than compete for a driver's conscious attention with yet another sign, the fix worked directly on the sensory input driving their instinctive reaction.

the move

Chicago's transportation engineers painted a sequence of white bars across the traffic lanes approaching the curve, spaced roughly 16 feet apart and narrowing progressively to about 8 feet apart as the curve got closer. A driver passing over evenly spaced bars at a constant speed perceives the bars flashing past faster and faster as the gaps shrink — the same sensory cue the brain uses to detect actual acceleration — so drivers instinctively braked to keep their perceived speed constant, even though their actual speed hadn't changed at all before they reacted. The treatment was later paired with flashing warning beacons and chevron signage.

why it works

The human visual system estimates speed partly by how quickly evenly spaced objects pass by in the field of view, and shrinking the gaps between the painted bars as a driver approaches the curve produces exactly the visual signature of acceleration, triggering an instinctive braking response before the driver consciously processes anything at all. Because the reaction is sensory and automatic rather than something a driver has to read and decide to act on, it operates in the same split-second window where a sign would typically be ignored or arrive too late to matter, which is precisely the window where crashes at a sudden curve actually happen.

the payoff

City traffic engineers reported a substantial drop in crashes at the curve in the months following the installation, and officials said the optical-illusion approach, already used with reported success on roads in Virginia, Texas and New York, might be extended to other high-accident locations across Chicago if it kept proving effective on Lake Shore Drive. The Oak Street S-curve nonetheless remained a persistently dangerous stretch of road years later, prompting continued city and state planning efforts to physically redesign the curve itself.

where it breaks

The effect depends on drivers not already knowing the trick — a driver who becomes consciously aware the bars are an intentional illusion, rather than a genuine visual cue their brain processes automatically, can learn to discount it, the same way repeated exposure can dull the effectiveness of any perceptual nudge over time. It also only addresses drivers misjudging their own speed; it does nothing for crashes caused by other factors like distraction, impairment, or genuinely reckless intentional speeding, and it can't substitute for addressing a road's underlying geometry if that geometry is the deeper cause of the danger, which is why Chicago pursued a physical redesign of the curve in parallel years later.

what came after

The painted-bar treatment at the Oak Street curve became a widely cited example in traffic-safety and behavioral-design circles of using a perceptual illusion rather than an explicit instruction to change driver behavior, referenced in discussions of similar techniques used elsewhere, including Thaler and Sunstein's book Nudge. Chicago and Illinois transportation officials continued planning a more permanent, physical redesign of the same curve years later, treating the optical-illusion markings as a low-cost mitigation rather than a full solution to the location's underlying danger.

references

1. [1]Optical illusion to lower speed on curveUPI, 2006upi.com
2. [2]How Road Designers Are Manipulating Us into Being Safer DriversCar and Driver, 2018caranddriver.com

keep it

same kind of clever

• 1911 · Aalsmeer Flower Auction (Royal FloraHolland)Aalsmeer sells its dying flowers by racing a price downward instead of bidding one upward
• 1911 · Roald Amundsen's South Pole expeditionAmundsen marked his supply depots sideways to the route, not along it, so a lost party could still walk into one
• 2017 · Behavioural Economics Team of the Australian Government (BETA) / Australian Taxation OfficeAustralia's tax office cut inflated deduction claims by writing to the accountant, not the taxpayer