U.S. Department of Transportation
Federal Highway Administration
400 Seventh St., S.W.
Washington, D.C. 20590

September 30, 2005
Refer to: HOTO-1

Mr. Keith Golden
State Traffic Safety and Design Engineer
Georgia Department of Transportation
935 East Confederate Avenue, Suite 300
Atlanta, GA 30316

Dear Mr. Golden:

Thank you for your September 28 email to Mr. Scott Wainwright of this office, requesting an official interpretation of the Manual on Uniform Traffic Control Devices (MUTCD) regarding ramp metering signal displays for a two-lane freeway entrance ramp with staggered release. You asked whether Chapter 4H of the MUTCD requires two signal faces for the entire ramp or two signal faces for each separately-controlled lane on the ramp. You also asked whether all the required ramp metering signal faces may be mounted overhead.

Ramp metering systems on two-lane entrance ramps use either "simultaneous release," in which both lanes receive the green indication concurrently and both lanes are stopped concurrently, or a "staggered release," in which one lane receives the green while the other lane is stopped and then the other lane receives the green while the first lane is stopped. When staggered release is used, the question arises regarding whether two signal faces per ramp (one face per lane) is adequate or whether two faces per lane should be used.

Section 4H.02 states that ramp control signals shall meet all of the standard design specifications for traffic control signals except as noted otherwise in that section. One of the specific exceptions is "a minimum of two signal faces per ramp shall face entering traffic." It is our interpretation that this MUTCD text assumed a one-lane ramp or a simultaneous release on a two-lane ramp and did not contemplate the use of a staggered release operation, which has come into use only relatively recently.

The traffic control signal design requirements contained in Chapter 4D require a minimum of two signal faces per approach. This requirement is based on the need for signal visibility when trucks or other conditions obscure the view of one face and the need to ensure the operation of at least one signal indication in the case of burnouts or other failures in one signal face. When a separately controlled turn movement is included in the signal phasing, such as a protected-only mode left turn, at least one additional separate signal face is required for that separate movement. Section 4D.15 states that, if the turn movement is the major movement from an approach, two left turn signal faces should be provided.

Since each of the two lanes on the ramp are separately controlled and receive the "go" and "stop" indications at separate times in a staggered release operation, this is comparable to the situation of the separately-controlled turn movement at a signalized intersection. Since the two ramp lanes are presumed to be approximately equal in volume and importance, each of the two lanes should be provided with two signal faces. This display assures that at least one signal indication will always be provided to each separately controlled lane in the event of failure or knockdown of one of the two signal faces, and it provides improved signal visibility for approaching traffic in each lane as well as the first vehicles in the queues.

Neither Chapter 4H nor Chapter 4D specifically require overhead mounting of signal faces. The Option statement in Section 4H.02 refers to allowable heights of signal faces "if located at the side of the ramp roadway." In practice, many agencies operating ramp metering systems have found it desirable to have one signal face low-mounted (4.5 to 6 feet above the ramp pavement) at the side of the ramp and one signal face mounted higher, either over the ramp or at the side. However, either overhead or side mounting of ramp metering signal faces is acceptable. The signal faces must be a minimum of 8 feet apart horizontally (Section 4D. 15). Some States using staggered release have used signs similar to the R10-10 (Left Turn Signal) sign to more clearly identify which signal faces control which ramp lane. These signs use the legends "LEFT LANE SIGNAL" and "RIGHT LANE SIGNAL".

Thank you for writing on this subject. If you have any questions, please feel free to contact Mr. Wainwright at scott.wainwright@thwa.dot.gov or call him at 202-366-0857. Please note that we have assigned your request the following official interpretation number and title: "4-294(I)-Ramp Metering Signals for Staggered Release." Please refer to this number in any future correspondence on this matter.

Sincerely yours,

Regina S. McElroy
Director, Office of Transportation
Operations

cc: Mr. Roger Wentz, ATSSA