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

A notorious 90-degree curve where Lake Shore Drive meets Oak Street in Chicago kept causing crashes no matter how many speed-limit signs the city posted nearby. Drivers approaching the curve set their speed by feel — how fast the road felt like it was moving beneath them — rather than by consciously reading and processing a sign in the fraction of a second before they needed to react, so posted limits and warning signs had limited effect on actual driving speed at the exact moment it mattered most.

The standard traffic-engineering response to a dangerous curve is more explicit warning: lower the posted speed limit, add more signs, install more visible signage. Chicago had already tried variations of this at the Oak Street curve without solving the underlying problem, because none of it changed what a driver's own senses were telling them about how fast they were actually going — the information a driver's foot on the accelerator was actually responding to.

Painted bars with shrinking gaps toward the curve, so drivers' visual speed sense reads acceleration and triggers instinctive braking before conscious reaction.

Drivers set speed by perceived motion, not by reading signs.

Narrowing bar gaps mimic acceleration, triggering instinctive braking.

The reaction is automatic, working in the split-second before a sign would register.

When people act on feel rather than instructions, change the feel, not the instructions.

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.