Eleven Weeks, Then Nothing

A Staggered-Adoption Difference-in-Differences Analysis of Driver-Feedback Speed Signs Across 61 Residential Streets, Against a Rotation Interval Set by Trailer Availability

A paper from the School of Continuous Improvement estimates what a council's rotating fleet of driver-feedback speed signs achieves across a full deployment rather than its opening fortnight. Sixty-one residential streets received a trailer-mounted sign between March 2023 and February 2026 in an order set by trailer availability, with permanent tube counters recording speeds throughout. Group-time treatment effects estimated against not-yet treated streets show mean speed falling 2.41 km/h in the first fortnight, attenuating monotonically, and reaching a precisely estimated null from week eleven (-0.09 km/h, 95% CI -0.38 to 0.20). The fleet's median deployment duration is 11.6 weeks, a figure set by the depot's rotation roster, so each sign is relocated within a fortnight of the point at which its effect stops being measurable. An expression module fitted to fourteen deployments accounts for 0.11 km/h of the effect and is retained for completeness.

@misc{slop_07lgun,
  author       = {Torun Ezeigwe and Thandiwe Solberg and Joost Nwosu},
  title        = {Eleven Weeks, Then Nothing: A Staggered-Adoption Difference-in-Differences Analysis of Driver-Feedback Speed Signs Across 61 Residential Streets, Against a Rotation Interval Set by Trailer Availability},
  year         = {2026},
  publisher    = {Slop University},
  doi          = {10.5555/slop.07lgun},
  url          = {https://slop.university/outputs/slop-paper-the-flashing-roadside-07lgun/},
  version      = {1.0},
  note         = {Research paper},
}