NCHRP-350 (NATIONAL COOPERATIVE HIGHWAY RESEARCH PROGRAM REPORT 350)

WHAT IS NCHRP 350?
Procedures are presented for conducting vehicle crash tests and in-service evaluation of roadside
safety features or appurtenances. The features covered by these procedures include
(1) longitudinal barriers such as bridge rails, guardrails, median barriers, transitions, and terminals;
(2) crash cushions;
(3) breakaway or yielding supports for signs and luminaries;
(4) breakaway utility poles;
(5) truck-mounted attenuators; and
(6) workzone traffic control devices

The purpose of the procedures is to promote the uniform testing and inservice evaluation of roadside safety features so that highway engineers may confidently compare the safety performance of designs that are tested and evaluated by different agencies. These procedures are presented in the form of guidelines that describe how a feature should be tested and evaluated. The identification of specific new, existing, or modified features for testing and evaluation; the selection of a level of service that the feature is to meet; and the establishment of acceptable performance criteria for an agency are policy decisions that are beyond the purview of this document.

The procedures are directed at the safety performance of roadside safety features; other service
requirements such as economics and aesthetics are not considered here. These procedures are
devised to subject roadside safety features to severe vehicle impact conditions rather than to
typical or average highway situations. Although the innumerable highway-site and safety-feature
application conditions that exist are recognized, it is impractical or impossible to duplicate these in a
limited number of standardized tests.

Hence, the approach has been to normalize test conditions: straight longitudinal barriers are tested although curved installations exist; flat grade is recommended even though installations are sometimes situated on sloped shoulders and behind curbs; idealized soils are specified although roadside safety hardware are often founded in poor soil or frozen ground. These normalized test conditions have a significant effect on a feature’s performance but are of secondary importance when comparing results of two or more systems.

For vehicle crash testing, specific impact conditions are presented for vehicle mass, speed, approach angle, and point on the safety feature to be hit. Standard test vehicle types are defined for minicompact and subcompact passenger cars, standard ¾-ton pickup trucks, single-unit trucks, and tractor-trailer cargo trucks. Impact speeds range from 35 to 100 km/h (approximately 20 to 60 mph), and approach angles vary from 0 to 25 degrees.

Three primary appraisal factors are presented for evaluating the crash test performance: structural adequacy, occupant risk, and after-collision vehicle trajectory. Depending on the safety feature’s function, it should contain, redirect, permit controlled penetration of the impacting vehicle, or permit a controlled stop in a predictable manner to satisfy structural adequacy requirements. Occupant risk relates to the degree of hazard to which occupants in the impacting vehicle would be subjected. It is measured in terms of the velocity at which a hypothetical unrestrained occupant strikes some part of the vehicle interior such as the instrument panel, window, or door, and the subsequent occupant ride down accelerations.

The after-collision vehicle trajectory is assessed based on the probable involvement of other traffic due to the path or final position of the impacting car and on the chances of undesirable post-impact vehicle behaviors such as pocketing or snagging. It is recognized that vehicle crash tests are complex experiments and are difficult to replicate because of the imprecise control of test conditions and the sometimes random and unstable behavior of dynamic crush and fracture mechanisms. Accordingly, care should be exercised in interpreting the results.

In-service evaluation is used in the final stage of development of new or extensively modified roadside safety features and has the purpose of appraising actual performance during a broad range of collision, environmental, operational, and maintenance situations for typical site and traffic conditions. This report updates the guidelines for in-service evaluation first provided in NCHRP Report 230, recognizing the complex nature of vehicular accidents and the limited resources of agencies responsible for monitoring the performance of new or modified safety features.

Appendices to this report provide (a) a commentary on the basis for the procedures; (b) soil specifications for the installation of safety features; (c) specifications for the instrumentation of the tests; (d) a summary on the analytical and experimental tools that can be used in the research and development phase for safety features; (e) a methodology for quantifying the damage to the occupant compartment of a test vehicle; (f) a description of calculation procedures for alternate measures of occupant risk (e.g., THIV, PHD, and ASI); and (g) a methodology for conducting and evaluating side impact tests.