Safety

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Transition to Texas T101 Bridge Rail Crash Tested

A transition from a W-beam on strong post guardrail to the Texas Type T101 bridge rail was recently crash tested with a 2,000-kg pickup truck. The purpose of the test was to determine whether this transition meets the strength requirements for Test Level Three of the National Cooperative Highway Research Program (NCHRP) Report No. 350. The pickup truck impacted the transition at 100 km/h and an impact angle of 25 degrees. It was smoothly redirected and remained upright. Damage to the transition was relatively minor.

Transition after crash test with a pickup truck.

-- Charles McDevitt (202) 493-3313

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Composite Traffic Barrier Tested

A composite traffic barrier (composite rail system) was tested recently at FHWA's Federal Outdoor Impact Laboratory (FOIL). The tests were performed using the FOIL's large outdoor pendulum and roughly emulated a small to mid-size automobile colliding with the composite rail system at a speed of 100 km/h and an approach angle of 20-25 degrees. The composite rail sections tested were hand fabricated and composed of a fiberglass composite material.

Why are composite materials such as fiberglass important to FHWA? Fiberglass is a composite material in which long glass fiber strands are imbedded in a surrounding plastic material. Today, fiberglass is used exclusively in the fabrication of recreational boat hulls and is also used in various automotive applications including crashworthy front ends. The glass fiber strands give the material great strength and the surrounding plastic material is used to bind the glass-plastic combination together as one material system. The advantages of composite materials such as fiberglass over the common construction materials of today (steel, concrete, and wood) are several. One is the material's innate resistant to corrosion. Another is the material's internal structure which can be custom fabricated to develop high strength in needed directions with attendant superior impact performance.

Composite rail tested for energy absorption by FOIL pendulum.

Composite materials do not have strength and stiffness properties that are the same in all directions (they are not isotropic), and, as such, they are much more difficult to analyze for potential design applications. This is one reason why composite materials have not been widely used in the transportation infrastructure to date.

This composite rail system development is part of an on-going cooperative agreement between FHWA and the Catholic University of America. Creative Pultrusions, Inc. is the composite material supplier and the tentative manufacturer of the rail system. Additional research on this composite rail is required. The composite rail system is far from finalized. Optimization of the rail's cross-section to insure maximum energy absorption, optimization for manufacturing, subsequent production of manufactured prototypes, additional pendulum testing, and full-size vehicle crash testing to determine the system's crashworthiness lay ahead.

-- Martin W. Hargrave (202) 493-3311

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Beware the Ides of March - Or -
Do Your Sunglasses Pass Muster?

In the days of the Roman Empire visibility might be diminished by sand in ones' eyes or by arrows coming at you from the Huns. Nowadays, while driving on our roads in an air-conditioned vehicle, visibility constraints are not generally expected except by environmental conditions. However, during the Ides of March (the early to middle of March, according to the Roman Calendar), we were alerted about a visibility problem with variable message signs (VMS) using light emitting diode (LED) technology for drivers wearing certain types of sunglasses. Smelling an intriguing opportunity to show our investigative prowess, we put on our Sherlock Holmes hat and, with the help of Dr. Watson, tried to get to the bottom of things!

We soon determined that the sunglasses in question attenuated very strongly in the visual spectrum where yellow/amber LEDs emit their visible energy. Not many types of sunglasses exhibit this characteristic, but those which do are excellent at making the messages of VMSs, using yellow/amber LEDs, almost invisible on a bright sunny day.

Our friend Sherlock secured samples of the sunglasses and LEDs. The Photometric and Visibility Laboratory has a spectroradiometer which can measure the spectral emittance of a light source as well as the spectral transmittance of a filter, i.e. the lens of sunglasses. Using this instrument, we quickly realized that the sunglasses in question severely attenuated light of wavelengths between 580 and 600 nm. The yellow-amber LEDs emit most of their energy around 590 nm. Putting these two findings together, we deduced why one of the engineers from the Maryland State Highway Administration thought the newly installed VMS was defective.

While these effects can be very dramatic on a bright, sunny day, the impact is not quite as severe on an overcast or rainy day. The sunglasses investigated attenuate the LED signal the same in sunny or rainy weather, but they do not attenuate the brightness of the surrounding landscape to the same degree. This causes the contrast between surroundings and the sign itself to change with changes in ambient lighting conditions.

Watson and Holmes did a little more investigating! First, they determined that the LEDs in question were within the color boxes specified by the Institute of Transportation Engineers (ITE). Then they found an American National Standard (ANSI Z80.3-1986) covering the optical performance of nonprescription sunglasses. Since this ANSI standard is based on a broad-spectrum light source, all the sunglasses we investigated met the ANSI requirements.

Dr. Watson turned these findings over to the Sunglass Association of America, the manufacturers of VMS, and to ANSI. ANSI is currently revising Z80.3-1986 and, hopefully, will consider narrow-band light sources, such as LEDs and certain types of lamps used for tunnel lighting, in their standards revision. The sunglass manufacturers have alerted their members to the problem.

Since LEDs save energy and require less maintenance, we can expect the use of LEDs in more and more applications. Hopefully, these investigations have alerted all parties involved in this project. With an ever changing technology, it is utterly important to be aware of developments and applications of new devices designed to improve safety while at the same time cutting energy consumption and maintenance costs. Keeping lines of communication open between manufacturers, standards writing organizations, and governmental agencies should be, as Sherlock Holmes would say, "elementary, my dear Watson!"

-- John Arens (202) 493-3364