Salt Lake City police officer Lester Wire invented the electric traffic light in 1912. Five years later, the city connected the lights of six intersections, which were controlled simultaneously from a manual switch.

Traffic engineers are up in arms about a Federal Highway Administration (FHWA) proposal to require 12-inch-diameter signal indications in virtually all new installations.

“This code change would make it impossible for cities to continue using historic signals in their historic districts,” says Barry Williams, founder of the American Streetscape Society.

No one knows how many of the nation's 272,000 traffic lights use 8-inch lamps, but they're not just specified for historic districts. More than 90% of New York City's 12,000 intersections, for example, have 8-inch lamps, according to New York City DOT Director of Signal Engineering and Street Lighting Alan Borock.

FHWA says that doubling the size of the lamps will make them more visible from a greater distance, thus lowering the likelihood of accidents—a belief that research supports. Right-angle accidents, for example, fell 47% after the city of Winston-Salem, N.C., replaced 8-inch with 12-inch signal indications on at least one approach at 55 locations throughout the city, and many departments have eliminated the smaller indications entirely.

However, replacing the lamps has implications beyond the cost of new equipment. Because moving from 8- to 12-inch lamps lengthens the signal head by 1 foot, mast arms may have to be raised to accommodate clearance requirements. With signs competing for space and placement at complex intersections, even a slight change in signal size may require the rearrangement of visual prompts.

As a result, at least one city is going to propose allowing the smaller lamps in certain speed limits. The FHWA requires 12-inch lamps whenever speeds are 40 mph or higher. Brian Kemper, signal engineer for Seattle, may recommend sticking with the 30 mph required in the city's historic Pioneer Square district.

To minimize the financial impact of its proposed change, the agency would allow traffic departments to hang onto their existing 8-inch signals for the rest of their useful life. Then, 12-inch indications would be required in all but the following:

  • Flashing yellow or steady green signal indications in an emergency-vehicle traffic control signal
  • Signal faces controlling the approach to a downstream location where two adjacent signalized locations are close to each other and it is not practical—because of factors such as high approach speeds, horizontal or vertical curves, or other geometric factors—to install visibility-limited signal faces for the downstream approach
  • Supplemental signal faces installed for the sole purpose of controlling pedestrian movements (see Section 4D.03 in the proposal) rather than vehicular movements.
  • Save the signals

    Share your opinion by the July 31 deadline.

    The Federal Highway Administration prefers that comments regarding its proposed amendment to the Manual on Uniform Traffic Control Devices be submitted electronically. To do so:

  • Go to
  • Find the teal-colored tab “More Search Options” at right; below it, click “Advanced Docket Search.”
  • Select “Federal Highway Administration” from the drop-down menu to complete the “Agency” field.
  • In the “Docket ID” field, type “FHWA-2007-28977” and click “Submit” at the bottom of the page.
  • Click the blue “FHWA-2007-28977” link at left (the only item in the “Docket” column).
  • Congratulations, you've arrived! Scroll down to “documents.” The first document listed is the proposed revision: To view it, go to “Views” at right. To comment, go to “Add Comments” at the far right and click the yellow icon. Complete the online submitter's form. (Tip: Omit the period after your middle initial because the field accepts only one character.) All other documents are public comments.