Atherton council introduces ordinance to codify local speed limits; staff to add 85th-percentile data for second reading

The Town of Atherton City Council on Sept. 17 introduced by title, and waived the first reading of, an ordinance to add chapter 10.6 to the Atherton Municipal Code to formalize local speed limits established by engineering and traffic surveys. The council voted unanimously to introduce the ordinance by roll call.

The ordinance action implements procedures required by the California Vehicle Code and the California Manual on Uniform Traffic Control Devices for establishing speed limits based on an engineering and traffic survey—commonly referred to as the 85th‑percentile speed. Town staff said the surveys determine the speed at which approximately 85% of vehicles travel at or below, and that the Vehicle Code requires rounding that measured value to the nearest 5 mph and provides limited discretion to reduce the posted limit in specific circumstances.

Staff identified two road segments recommended for a reduced speed limit tied to prior council direction: Middlefield Road, and Valparaiso Avenue between El Camino Real and Cotton. Staff also pointed out a typographical error in the staff report and draft resolution that transposed speed segments for part of Atherton Avenue; the council asked staff to correct that at the second reading. Council Member DeGolia asked that staff include the measured 85th‑percentile speeds for every street listed when the ordinance returns for final action; staff agreed to add that table.

Council and staff discussed data-collection windows and methodology. Staff said traffic counts are collected in free‑flow conditions (outside peak commute and school pickup/dropoff) and that surveyors collect either a minimum of 100 vehicles or one hour of observations, whichever occurs first. Council members pressed that, for the Active Transportation Plan and for school‑adjacent streets, supplemental counts of bicycles and pedestrians (and a 12‑hour camera window used by the data collector) were also taken and would inform the town’s active‑transportation work but do not replace the traffic‑survey methodology for setting vehicle speed limits.

No change to the town’s existing speed limits was proposed except for the two reductions noted. The council unanimously approved introducing the ordinance and asked staff to return with the full 85th‑percentile data and corrected language at the second reading.

The ordinance will return to a future council meeting for adoption after staff incorporates the requested data and corrects typographical errors.