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We’ve gotten some questions about the use of gravel/sand barrel array configurations for speeds above 62 mph.  We use a 19-barrel array for “TL-3” applications.  One of our vendors shows 19 barrels as a 70 mph configuration, one shows it as a 65 mph configuration, and the last won’t show anything but the 12-barrel TL-3 tested configuration in their literature.

We have a desire to keep our larger arrays for high speed interstate applications and want to better understand the origins of the higher speed configurations and assess the risk of their continued use.  From what I can find the configurations were inertia calculations(?) under NCHRP 350. 

I took a look at the INDOT PCB arrays. Results can be seen in the files at the link below. We have pretty advanced inertial barrel analysis procedure that I can use to look at your arrays. When we analyzed the array, we basically have three criteria. First, the vehicle must drop its velocity below 10 mph by the end of the array. Second, the calculated OIV must be below 40 ft/s. Third, the calculated average deceleration must remain below 12 g’s. The average acceleration does not capture peaks in the acceleration like an actual impact event, so the acceleration threshold is lowered from 20 g’s to 12 g’s. This is consistent with section 8.4.3 in the RDG. Our analysis is just a more advanced version of that procedure.

MASH has 5 tests for a no-redirective crash cushion like sand barrels. I did not analyze the 3-45 test for the 1500A sedan based on the successful analysis of the 1100C and 2270 P vehicles. I can if you like. I also did not analyze test nos. 3-42 and 3-43, which are angled impacts on the nose of the system. For your array with a single barrel in front, these are non-issues.

I analyzed your proposed TL-3 array for end on impacts with an 1100C vehicle and 2270P vehicle at 62 mph and 70 mph. In all 4 impacts, the TL-3 array worked well and met all of the criteria.

I also conducted analysis of impact on the side of the system. I did both test no. 3-44, the coffin corner impact, and another impact farther up the side of the system. Th3 3-44 test is only with the 2270P vehicle in MASH. I only analyzed the 1100C for the impact farther upstream on the side of the system as it was more critical and if it worked, the pickup truck would be assumed to work as well, though it would likely gate through the cushion. I should note that these analyses are not typically conducted on inertial barriers. The procedures used by the manufacturers and the RDG have not typically looked at oblique impacts.

The 1100C impact on the side of the system worked fine at both speeds. No issue there and that is not a requirement, but we do like to check it. The 3-44 test analysis looked ok. No issues with acceleration or OIV. I should note that the vehicle velocity was not below 10 mph at the end of the array ( it was 14.1 mph). This is slightly higher than the threshold. However, I looked at the two barrel systems that have MASH letters. Both of these systems have less mass in the barrels impacted in the 3-44 coffin corner impact. As such, we would expect lower velocity drops and higher final velocity at the end of the array in those systems and they both met MASH. As such, I would believe that your array would work as well for TL-3 impact conditions. I ran the 3-44 analysis at 70 mph as well. It performed similarly, but it had a residual velocity of 15.9 mph. Again, that may not be an issue given the performance of existing crash tests.

All of the analysis is in the link. If you need more information or want to discuss, let me know.

Thanks!