View Q&A

In our role as General Engineering Consultant to the Illinois Tollway, we are tasked with keeping the Tollway Standards up to date.  The Tollway has adopted the Illinois Department of Transportation's Highway Standard for standard w-beam guardrail.  IDOT recently revised their standard to use the Midwest Guardrail System, which has the higher rail and 12" block-outs.  IDOT allows the use of plastic block-outs and considers them equivalent and interchangeable with wood.  They also allow plastic and wood to be intermixed within a run of guardrail.

The FHWA also considers the "crashworthy" plastic block-outs to be interchangeable with wood. That means they need a FHWA acceptance letter before they can be used.

The Tollway does not have tort immunity, and therefore is very cautious about safety devices.  Currently, the Tollway does not mix and match and does not allow plastic block-outs.  The Tollway's policy is to only replace in kind when guardrail is in need of repair.  There is concern with UV deterioration and stocking of many types of blocks because all maintenance is done by Tollway forces.

Has there been any concern over UV deterioration for plastic blocks?

The FHWA sent me an approval letter for the "Monroeville 12 inch MGS Composite Offset Block" dated April 16, 2008.  To your knowledge has any other product been approved or crash-tested using the larger 12" plastic block-out?

Thank you for your inquiry regarding the use of plastic blockouts. For many years now, several companies have developed various compositions of recycled blockouts for use in strong-post, W-beam guardrail systems. Based on my understanding of these blockouts, most would contain additives that inhibit UV degradation, such as the use of carbon black in tires. However, if you were concerned with UV degradation, it would be recommended that you ask a potential supplier/manufacturer whether resistance to UV deterioration is provided.

With regard to the use of plastic and wood blockouts in standard, W-beam guardrail systems, I am not concerned with using multiple, approved blockouts within the same longitudinal barrier system. Although this type of configuration may be rare in new construction, it may be more common during guardrail maintenance operations over time. In any event, the safety performance of the barrier system should not be affected with the use of different blockout materials that have been approved for use in the barrier system.

I am familiar with the research report which documents the crash testing of the MGS using a 12" deep composite blockout. Unfortunately, I am not aware of any other 12" deep plastic blockout that has been tested, evaluated, and approved for use with the MGS. However, there is new variation of the MGS which utilizes round wood posts with 12" deep wood blocks adapted to the round posts.

Finally, other 31" tall W-beam guardrail systems have been developed to date and have included options for use with and without blockouts. If further information is required on these systems, I recommend that you review the FHWA website for approved crashworthy hardware as well as the corporate websites for NUCOR-Marion, Trinity Industries, and Gregory Industries.