Wednesday, September 22, 2021 8:39 pm

The Monday Morning Commute: Ask the traffic engineer; Speed limits?

By Paul Basha, traffic engineer, Summit Land Management

How and by who are speed limits on public roads established ?

Paul Basha of Summit Land Management has made several of such decisions as a public official and has also recommended speed limits as a consultant to public agencies. Furthermore, through undergraduate and graduate courses at Arizona State University or at traffic engineering workshops, Paul has taught others the process, who now either determine or recommend public road speed limits. 

The Manual on Uniform Traffic Control Devices, published by the Federal Highway Administration, provides the criteria for establishing speed limits. (Lest you think this is an obscure meaningless document, this book requires that red traffic signals mean stop and green signals mean go. The book also requires stop signs to have their unique octagonal shape with a red background and white letters. Among many other traffic laws, the book also mandates that yellow stripes mean opposite direction traffic on the other side of the line – with two exceptions – and white stripes mean same direction traffic on the other side of the line.) 

The Arizona Revised Statues, Section 28-702.04 requires local authorities to provide “an engineering and traffic investigation” to determine posted speed limits – understood to be in conformance with the Manual on Uniform Traffic Control Devices requirements. 

Essentially, the manual requires the local agency to measure prevailing travel speeds. The posted speed limit must be the closest 5-mile-per-hour increment to the speed that 85% of the vehicles are traveling or below. The speed limit can be lower only if “an engineering and traffic investigation” discovers that curves or hills, or intersections, or pedestrians and parking presence require a lower speed limit.   


Call or e-mail Paul at (480) 505-3931 and

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February 2020