The Delaware Manual on Uniform Traffic Control Devices (MUTCD) is a highly prescriptive document with a focus on uniformity and clear and concise messaging. A key requirement for signs is retroreflectivity and the 2011 MUTCD quantified the minimum maintained retroreflectivity levels for most signs. These requirements are found in Part 2 of the Manual at Sections 2A.07 and 2A.08.
Standard:
02 Regulatory, warning, and guide signs and object markers shall be retroreflective (see Section 2A.08) or illuminated to show the same shape and similar color by both day and night, unless otherwise provided in the text discussion in this Manual for a particular sign or group of signs.
03 The requirements for sign illumination shall not be considered to be satisfied by street or highway lighting.
While there are some illumination methods (other than street or highway lighting, which don’t suffice), the Standard generally is satisfied by the use of retroreflective sheeting. Let’s geek out on that for a moment to understand what is meant by retroreflectivity. For reflectivity, think of a mirror. If you stand directly in front of a mirror, you see yourself and can reflect on your own magnificence, but if you move to one side and look at the mirror, you see an area off to an opposite angle from your own angle to the mirror, such as when using your vehicle’s side view mirror. For matte reflectivity, think of a painted wall. Depending upon its finish (flat, glossy, semi-gloss, eggshell), very little light is reflected (or only part of the color spectrum; let’s not go too far down that rabbit hole). In the case of retroreflectivity, the surface material returns a large portion of the directed light back in the direction of its source. In the case of sign sheeting, this was originally done with glass beads embedded on the surface, but the technology has advanced to the use of prismatic structures on the surface and even beyond. Increasingly, various forms of prisms and cubes have been used so that the light bounces within the surface to an opposing face of equal angle, sending it back out in direction from which it came.
The results are signs that are highly visible at night. But the best of materials will degrade with time, as seen in the test stand shown here, and they must be replaced before they fall below required levels of retroreflectivity for various colors, found in Table 2A-3. Now, your first viewing of Table 2A-3 can be a bit daunting, but it’s not as scary as it first appears. Since beaded sheeting is used less and less, you can focus on the prismatic column and the W, G, R, O, and Y are just shorthand for white, green, red, orange, and yellow background colors.
Click to enlarge
The unit of measure is candela per lux per square meter, cd/lx/m2. Candela is the unit of luminous intensity and lux (one lumen per square meter) is a measure of luminous flux per unit area (in this case, square meters). They are weird units, but we don’t need to worry much about them, other than be aware of them. We could just as soon call them Fred Units – you need 50 Fred Units for the white sheeting on a Speed Limit sign.
So to understand the basics of Table 2A-3, you need to think about the type of sign you have. For white on green signs, think of your Street Name signs. For black on yellow, think of Curve Ahead and other warning signs. For black on orange, think of Road Work Ahead and other temporary traffic control signs. For white on red, think of Stop or Yield signs. For black on white, think of Speed Limit and route signs. Note that black is not intended to retroreflect – it creates a contrast – so you won’t see a value for black.
By way of example, a post mounted white on green Street Name sign requires at least 120 cd/lx/m2 for the white (the legend) and at least 15 cd/lx/m2 for the green background. A Curve Ahead sign requires at least 50 cd/lx/m2 for the yellow background, but the additional criteria footnote increases this to 75 cd/lx/m2 unless the sign is one of the “Bold Symbols” or is greater than 48”. A Speed Limit sign requires at least 50 cd/lx/m2 for the white background.
Now, for white on red, there is an additional test. The red background must be at least 7 cd/lx/m2 and the white legend must be at least 35 cd/lx/m2, but the ratio of white to red must also be at least 3. For example, if the red is measured as 15 cd/lx/m2 and the white is measured as 40 cd/lx/m2, the ratio would be less than 3 and the sign would fail.
Take your time with Table 2A-3 and it will start to make sense. It is important because you must have a process in place to ensure compliance.
Standard:
02 Public agencies or officials having jurisdiction shall use an assessment or management method that is designed to maintain sign retroreflectivity at or above the minimum levels in Table 2A-3.
You may ask if this means that every one of your signs must meet these levels all of the time. Well, that is the goal, but it is not the Standard. The Standard requires you to have a process in place designed to maintain sign retroreflectivity. The Support statement hints at what is needed for compliance with the Standard.
Support:
03 Compliance with the Standard in Paragraph 2 is achieved by having a method in place and using the method to maintain the minimum levels established in Table 2A-3. Provided that an assessment or management method is being used, an agency or official having jurisdiction would be in compliance with the Standard in Paragraph 2 even if there are some individual signs that do not meet the minimum retroreflectivity levels at a particular point in time.
You may then ask what is meant by “some individual signs.” Is this 1% of your signs, 5%, 7%? We don’t know and the Federal Highway Administration will not say. If judged in court, you would be presumably be held to a “reasonable person’s standard.” It is fair to say that if negligence was alleged against your agency and 40% of your signs failed to meet Table 2A-3, your attorney would have a very difficult time defending you. Instead of worrying about exactly how many signs can be below the required levels, the Standard requires us to have a process that is designed to target zero.
The methods to measure sign retroreflectivity are described in Section 2A.08. You may directly measure the signs using a retroreflectometer, like the one shown here. These are expensive and required periodic calibration, but the Delaware T2/LTAP Center owns one and can assist you with it if you which to go this route. However, you can also just change signs out on a scheduled basis, using the Expected Sign Life of Blanket Replacement method. You can also use Control Signs, where sign stands in your maintenance yard, representative of batches of sign in the field, are measured with a retroreflectometer. Nighttime visual inspections by older drivers in specific types of vehicles with specific headlights can also be used.
Not all signs must adhere to quantitative standards at this time, according to this Option in Section 2A-08.
Option:
06 Highway agencies may exclude the following signs from the retroreflectivity maintenance guidelines described in this Section:
- Parking, Standing, and Stopping signs (R7 and R8 series)
- Walking/Hitchhiking/Crossing signs (R9 series, R10-1 through R10-4b)
- Acknowledgment signs
- All signs with blue or brown backgrounds
- Bikeway signs that are intended for exclusive use by bicyclists or pedestrians
Finally, the MUTCD recognizes that some roadway agencies don’t have engineers on staff or have engineers with sufficient experience and training with respect to roadway design or traffic control devices, and, “as part of the Federal-aid Program, each State is required to have a Local Technical Assistance Program (LTAP) and to provide technical assistance to local highway agencies. Requisite technical training in the application of the principles of the MUTCD is available from the State’s Local Technical Assistance Program for needed engineering guidance and assistance.”
The Delaware T2/LTAP Center’s Municipal Engineering Circuit Rider is intended to provide technical assistance and training to local agencies and so if you have MUTCD questions or other transportation issues, contact Matt Carter at matheu@udel.edu or (302) 831-7236.
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