To answer your question on the minimum length for a concrete barrier to meet TL-4 criteria, it all depends on what you want it to do. When we conduct crash testing to TL-4, we typically utilize a 100-150 ft long system to ensure the vehicle box has adequate time (system length) to stabilize following the impact event. After initial impact, the vehicle typically rolls toward the system and leans on the barrier. If the barrier is too short, the vehicle will continue to roll and may eventually rollover. A shorter system length may be able to contain a TL-4 single unit truck, but the stability and trajectory of the vehicle after impact is unknown.
Also, system lengths are typically calculated using run-out lengths to ensure the barrier extend far enough upstream of the hazard to prevent arrant vehicles from getting behind the barrier and impacting the hazard. You may want to consider this before selecting a very short barrier length.
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