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Interpretation Letter 4(09)-7(I) - Size of Flashing Beacons

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DOT Logo

U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Avenue, SE.
Washington, D.C. 20590

February 8, 2011

In Reply Refer To: HOTO-1

Sally S. Olsen, P.E.
Office of Traffic Safety & Mobility
New York State Department of Transportation
50 Wolf Rd
Albany, NY 12205

Dear Ms. Olsen:

Thank you for your January 4 e-mail to Mr. Scott Wainwright of our Manual on Uniform Traffic Control Devices (MUTCD) Team regarding the size of flashing beacons. You asked whether the requirement in the 2009 MUTCD that new vehicular signal faces shall be 12-inch diameter size (with certain exceptions) also applies to the size of flashing beacons as described in Chapter 4L of the MUTCD.

It is our Official Interpretation that the new 12-inch size requirement for vehicular signal faces is not intended to apply to the size of flashing beacons. Flashing beacons may be either 8 inches or 12 inches in nominal diameter. Our reasoning for this interpretation is as follows.

In the 2009 MUTCD, Section 4L.01 deals with the general design and operation applicable to all of the various types of beacons included in Chapter 4L. Paragraph 1 of Section 4L.01 indicates that flashing beacons are comprised of one or more signal sections operated in a flashing mode and Paragraph 2 states that flashing beacons and their mountings shall comply with the provisions of Chapter 4D unless otherwise provided in Chapter 4L. The only provision in Chapter 4L that deals with beacon size is Paragraph 2 of Section 4L.04, which states that Speed Limit Sign Beacons shall be "not less than 8 inches in diameter."

In Chapter 4D, Section 4D.07 governs the size of vehicular signal indications. Paragraph 2 of Section 4D.07 requires that 12-inch indications shall be used for all signal sections in all new vehicular signal faces, except for certain conditions as described in Paragraph 3, for which 8-inch indications may be used. None of those conditions in Paragraph 3 are written in a way that would address flashing beacons.

In reviewing the January 2, 2008, Notice of Proposed Amendments, the comments to the docket, and the Final Rule for the 2009 MUTCD, it is our conclusion that that the new provision in Section 4D.07 was intended to apply to vehicular signal faces only and that new flashing beacons were not intended to be required to be the 12-inch size. We believe that it was an inadvertent oversight that appropriate language to exempt flashing beacons from this requirement was not included in Chapter 4L and/or Section 4D.07. At its January 2010 meeting, the Signals Technical Committee of the National Committee on Uniform Traffic Control Devices discussed this matter, including their comments to the rulemaking docket for the 2009 MUTCD regarding the proposal for the 12-inch size for all vehicular signals, and that committee concurred with this conclusion.

Most flashing beacons are supplemental to signs rather than constituting the primary control of a location and most are post-mounted at the roadside rather than overhead. As such, size of the flashing beacon is not as important for visibility and conspicuity as it is for traffic control signal faces. We believe it is appropriate to continue to allow flashing beacons to be either 8 or 12 inches in diameter, with the decision on beacon size to be based on engineering judgment or study of the conditions at any given location, the type of beacon, and whether it is being mounted overhead or at the roadside.

Thank you for writing on this subject. We hope that our interpretation answers your question. For the next edition of the MUTCD, we will consider proposing revisions to the Sections 4D.07 and 4L.02 to formally correct this inadvertent error and clearly indicate that flashing beacons may be either 8 or 12 inches in diameter. If you have any questions, please contact Mr. Wainwright by e-mail at or by telephone at 202 366 0857. Please note that we have assigned your request the following official interpretation number and title: "4(09)-7 (I) – Size of Flashing Beacons." Please refer to this number in any future correspondence regarding this issue.

Sincerely yours,

Original signed by:

Mark R. Kehrli
Director, Office of Transportation Operations

Incoming E-mail

PDF Version, 48KB

From: Olsen, Sally (DOT)
To: Wainwright, Scott (FHWA)
Subject: beacon sizes
Date: Tuesday, January 04, 2011 10:51:17 AM

Hi Scott —

Some of us have conflicting understandings of what sizes are allowed for beacons. Section 4L.01 p02 says that beacons shall comply with 4D unless otherwise provided in 4L. Section 4L.04 p02 mentions that speed limit sign beacons must be at least 8" diameter, and there is no other text in 4L about size. This lack of any other 4L text about size suggests to some that all other beacons must be 12" according to 4D.07 p02 since beacons are not mentioned as any of the exceptions to the 12" size listed in 4D.07 p03. Others think that any beacons are still allowed to be 8".

Who is right — the "only SL beacons may be 8" " group. Or the "any beacons may be 8" " group?


Sally S. Olsen, P.E.
Office of Traffic Safety & Mobility
50 Wolf Rd
Albany, NY 12205