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Public Roads - Summer 1995

Metrication of Roadside Hardware

by Malcolm H. Ray

As a part of its continuing effort to standardize roadside hardware components, Task Force 13 of the Joint Cooperative Committee of the American Association of State Highway and Transportation Officials (AASHTO), American Road and Transportation Builders Association (ARTBA), and Associated General Contractors of America (AGC) recently updated its Guide to Standardized Highway Barrier Rail Hardware, which was previously published in 1979. The revised guide is now available from AASHTO.

This article summarizes some of the more important recommendations regarding metrication of roadside safety hardware.

One prominent feature of the revised guide is the exclusive use of the International System of Units (SI), a modernized metric system. The updated guide should prove to be a valuable resource for states in converting their roadside hardware standards to SI prior to Sept. 30, 1996 -- the deadline for submitting federal-aid designs in SI units.

The 1979 guide contained information only on highway barrier components with just a few drawings of complete guardrail systems. The revised guide has drawings and specifications of approximately 90 proprietary and non-proprietary systems and their components, including system drawings of:

  • Guardrails.
  • Median barriers.
  • Bridge railings.
  • Guardrail terminals.
  • Guardrail-to-bridge-railing transitions.
  • Crash cushions.
  • Work-zone barriers.

In general, the component dimensions shown in the revised guide were converted to SI by "soft" metric conversion. A soft conversion is a direct mathematical conversion from a U.S. Customary Unit (USCU) to its metric equivalent.

This means that the SI values represent the same physical shape as the USCU dimensions within the manufacturing tolerance. Thus, the physical shape of common hardware items such as the W-beam and thrie-beam guardrail cross section are the same as those currently in use.

The intent of the guide is to convert the values to SI units without creating a need for manufacturers to retool their production facilities and to ensure that SI hardware can be used interchangeably with the hardware already in place on the nation's highways.


The revised guide takes advantage of all of the most recent activities of standards-making organizations, such as the American Society for Testing and Materials (ASTM), American National Standards Institute (ANSI), and AASHTO. These organizations have already converted many of their more common material specifications to SI.

Perhaps the most notable change in the revised guide is the exclusive use of metric fastener standards. The M-profile threads described in ANSI B1.13M are the basis for all threaded parts in the revised guide. Specifications for a wide variety of bolts, screws, and nuts are given in ANSI B18. The Industrial Fastener Institute (IFI) publishes a very useful compilation of all the ASTM, ANSI, and IFI metric fastener specifications.

Both AASHTO and ASTM have been very active in converting their material specifications to SI. The converted specifications normally used the same AASHTO and ASTM designations with an "M" suffix. For example, standards for USCU billet reinforcing steel can be found in AASHTO M-31 (ASTM A615) and the SI equivalent can be found in AASHTO M-31M (ASTM A615M).

The specification of structural steels in the revised guide has also been updated to take advantage of the new AASHTO M-270M steel specifications. This one specification combines several formerly separate bridge steel material specifications into one. For example, AASHTO M-183M (ASTM A-36M) steel is now designated as AASHTO M-270M Grade 250, where the grade number refers to the yield strength in megapascals (MPa), and AASHTO M-222M (ASTM A-588M) corrosion-resistant steel is now designated as AASHTO M-270M Grade 50W.


Perhaps the most noticeable change resulting from the metrication process is the heights of common guardrails shown in figure 1. Specifying the height of a guardrail to the nearest millimeter suggests an inappropriately small construction tolerance.

The middle cable of a weak-post, three-cable guardrail is generally mounted 24 inches (in) above the ground. The exact SI conversion is 609 millimeters (mm), which is rounded to 610 mm. In the field, it is nearly impossible to measure the difference between a 609-mm-high post and a 610-mm-high guardrail. Post heights are generally converted to a value ending in zero since this results in whole-centimeter dimensions, which are convenient for construction workers.

The mounting height of strong-post guardrails provided the opportunity to standardize several guardrail heights. The rail-to-post mounting bolt at the center of a strong-post, W-beam guardrail is usually 21 in above the ground. The exact conversion suggests that a mounting height of 533 mm is appropriate for strong-post, W-beam guardrails. The center of a strong-post, thrie-beam barrier is usually 22 in above the ground, suggesting a height of 559 mm. Guardrails cannot be installed on a typical compacted shoulder to greater than ±½ in (±12 mm). Specifying 533 mm or 559 mm would suggest an inappropriately fine construction tolerance. The center-of-rail height of both W- and thrie-beam guardrails are standardized to 550 mm, increasing the W-beam by 17 mm (about 5/8 in) and decreasing the thrie-beam by 9 mm (about 3/8 in). In this case, metrication of the dimension permits the standardization of two formerly different systems to the same value.

A post spacing of 1905 mm for typical W-beam and thrie-beam guardrails is recommended. This value is the exact conversion of the 6-foot 3-in post spacing normally used today. While there is interest in using 2000 mm for a post spacing, maintenance and inventory problems preclude adopting it at the present time.

Bridge Railings and Concrete Median Barriers

Bridge railing heights of 27, 32, and 42 in have been used in a number of common bridge railings. SI values of 710 mm, 810 mm, and 1070 mm are used in the revised guide. The lower bridge railing height has been raised by 25 mm to ensure that where the guardrail top is at 706 mm, guardrail-to-bridge-rail transitions are not above the top of the bridge parapet. New 706-mm-tall guardrails will have to be vertically transitioned down 20 mm to ensure that the top of the guardrail is not above the top of the parapet.

Safety-shape profiles are a common feature of many bridge railings, median barriers, and construction work-zone barriers. There are two basic shapes used in the United States: the New Jersey shape and the F shape. The shapes, shown in figure 2, are basically the same as the current shape, except some of the values have been rounded to obtain more rational dimensions.


The SI values suggested in the revised Guide to Standardized Highway Barrier Hardware should cause little disruption in constructing and maintaining roadside hardware. The physical shape of nearly all hardware components is essentially the same as shown in the 1979 guide. In most cases, dimensions that were significantly changed usually were changed more in an attempt to standardize several similar pieces of hardware rather than to achieve rational SI units. The 1979 guide was an effective tool for standardizing roadside hardware; the revised guide should also help to promote standardization and ease the transition to SI.


  1. A Guide to Standardized Highway Barrier Rail Hardware, AASHTO-ARTBA-AGC Joint Cooperative Committee, Subcommittee on New Highway Materials, Task Force 13, 1979.
  2. W.A. Brenner. "Federal Highway Administration," Metric in Construction, Volume 3, Issue 1, Construction Metrication Council of the National Building Sciences, Washington, D.C., January 1994.
  3. Metric Fastener Guide, Second Edition, Industrial Fastener Institute, Cleveland, Ohio, 1983.
  4. A Guide to Standardized Highway Barrier Hardware, AASHTO-ARTBA-AGC Joint Cooperative Committee, Subcommittee on New Highway Materials, Task Force 13, 1995 (pending).

Dr. Malcolm (Mac) Ray is the president of Momentum Engineering Inc. He was the principal investigator in a National Cooperative Highway Research Program project to update the AASHTO-ARTBA-AGC Guide to Standardized Highway Barrier Hardware. He has more than a decade of experience in roadside safety research, including crash testing hardware, simulating collision events, and formulating test and evaluation criteria and design. He received his doctorate in civil engineering from Vanderbilt University. He is a licensed professional engineer in Tennessee.