February 27, 2003
Refer to: HSA-10/WZ-138
Marketing Displays International
38271 W. Twelve Mile Road
Farmington Hills, MI 48331-3041
Dear Mrs. Rogalla:
This is in response to your letter of November 20, 2002, requesting Federal Highway Administration (FHWA) acceptance of your company's portable breakaway sign stand, MDI Model 4884 as a crashworthy traffic control device for use in work zones on the National Highway System (NHS). Accompanying your letter was a drawing of the stand. You requested that we find this stand acceptable for use on the NHS under the provisions of National Cooperative Highway Research Program (NCHRP) Report 350 “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” You provided additional details at our request on January 30 and on February 21, 2003.
The FHWA guidance on crash testing of work zone traffic control devices is contained in two memoranda. The first, dated July 25, 1997, titled “INFORMATION: Identifying Acceptable Highway Safety Features,” established four categories of work zone devices: Category I devices were those lightweight devices which could be self-certified by the vendor, Category II devices were other lightweight devices which needed individual crash testing, Category III devices were barriers and other fixed or massive devices also needing crash testing, and Category IV devices were trailer mounted lighted signs, arrow panels, etc. The second guidance memorandum was issued on August 28, 1998, and is titled “INFORMATION: Crash Tested Work Zone Traffic Control Devices.” This later memorandum lists devices that are acceptable under Categories I, II, and III.
The MDI Model 4884 breakaway portable sign stand is similar to your crash-tested breakaway stand, the MDI Model 4860. The Model 4860 was found acceptable in our letter WZ-69, dated May 9, 2001. In your model numbers the first two digits identify the size of the sign that can be supported and the second two digits identify the mounting height, both in inches. The Model 4860 was described as follows:
Model 4860, with breakaway feature, tested with Aluminum panel and light:
This stand is manufactured with two vertically mounted steel coil springs to support 1219 x 1219 x 2 mm (48 x 48 x 0.080 inch) aluminum signs. The four legs of the stand are 32 x 32 x 2.5 mm (1.25 x 1.25 x 0.10 inch) square 6061-T6 extruded aluminum tubing. The coil springs attach the 5 mm (0.19 inch) thick autophoretic coated steel base to the telescoping upright. The two-piece upright, consisting of a 38 mm (1.50 inch)(bottom) and a 32 mm (1.25 inch)(top) square aluminum tube, supports the sign. The bottom upright has a scored “breakaway section” at 483 mm (19 inches) above grade when the stand is fully assembled. Rigid sign mounting brackets support the aluminum sign at top and bottom and a steel flag lock bracket is attached to the top upright supporting three hardwood dowel flags. A barricade light assembly, consisting of a 6-volt battery and a 178 mm (7 inch) polycarbonate lens, is mounted to the top upright above the flag bracket. The overall height of the stand, with flags and light is 4115 mm (162 inches) and 3251 mm (128 inches) without the light and flags. The bottom of the sign mounts to 1524 mm (60 inches) above grade. The total weight of the sign stand is 31.6 kg (69.7 pounds) which includes the stand of 21.7 kg (47.7 pounds), the sign and flags of 8.4 kg (18.5 pounds) and the light of 1.6 kg (3.5 pounds) as tested in MDI 4.
Your current request is for the Model 4884 breakaway portable sign stand. The Model 4884 is identical in construction to the Model 4860 with the exception of the mast, which is in three parts. The bottom section of the Model 4884's mast is 775 mm (30.5 inches) long of 44.5 mm (1.75 inch) square aluminum with a wall thickness of 2.54 mm (0.10 inch). The middle section is 865 mm (34 inches) long of 38.1 mm (1.50 inch) square aluminum with a wall thickness of 2.54 mm. The top section is 1725 mm (68 inches) long of 31.75 mm (1.25 inch) square aluminum with a wall thickness of 2.54 mm.
Testing and Findings
Full-scale automobile testing was conducted on the Model 4860 and was found acceptable. The Model 4884 is similarly constructed, includes the breakaway feature, and mounts the sign higher above the roadway. The only significant difference is the size of the lower mast, which is where the breakaway feature (external groove) is found. Because this could have a material effect on the breakaway performance of the stand, we asked you to conduct static tests to determine what forces were needed to activate the breakaway feature. When subjected to horizontal beam load tests, the 1 ½ inch specimen broke under a static beam load of 1480 pounds. The 1 ¾ inch specimen broke at a load of 1760 pounds. The difference of 280 pounds of force was less than a 20 percent increase. After reviewing the crash tests of the Model 4860 stand we conclude that any delay in the breakaway action of the larger mass would very likely be offset by the increased height of the sign and the sign and mass should still pass over the impacting vehicle. Therefore, the MDI Model 4884 breakaway portable sign stand described above and detailed in the enclosed drawing is acceptable for use on the NHS under the range of conditions that the Model 4860 was tested, when proposed by a State.
Please note the following standard provisions that apply to FHWA letters of acceptance:
Michael S. Griffith
Acting Director, Office of Safety Design
Office of Safety