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    The differences between NCHRP 350 TL-3 and TL-4

The differences between NCHRP 350 TL-3 and TL-4

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The U.S. National Cooperative Highway Research Program (NCHRP) 350 provides a uniform set of guidelines for conducting full-scale crash tests for permanent and temporary highway safety hardware. NCHRP 350 testing applies to road safety barrier hardware accepted by the New Zealand Transport Agency (NZTA) before November 2012.

NCHRP 350 performance criteria guidelines include the ability of the system to contain and redirect the crash energy, occupant risk and post impact vehicle trajectory. Performance levels are measured in relation to crash energy containment as defined in Test Levels (TL) 1 through 6. Crash energy is developed through the interaction of the vehicle mass, speed and angle of impact. It may be calculated using the following formula:

CE = 0.5M(Vsinθ)²
CE = Crash Energy
M = Mass
V = Speed
Θ = Angle of Impact

The TL-3 test vehicles are an 820kg small car and 2,000kg pickup truck. The small car is used to ensure that the barrier impact severity does not pose an excessive occupant risk. TL-4 adds an 8,000kg single unit truck test vehicle. Allthough TL-3 and TL-4 have different combinations of mass, speed and angle of impact, the maximum crash energy containment requirements are the same. However the maximum deflection, whether it be TL-3 or TL-4, should be used for design layouts.

Test Level Maximum Test Vehicle Mass Impact Speed Impact Angle Crash Energy
TL-3 Pickup - 2000kg  100kph 25 138.1kj
TL-4 Single Unit Truck - 8000kg 80kph 15 138.1kj

Higher centre of gravity vehicles will have a tendency to roll over a barrier system when impacting at shallow angles. This is usually managed by using higher barrier systems to resist the rollover.  The TL-4 8000kg single unit truck is used to assess the ability of the system to limit higher centre of gravity vehicles from rolling over the barrier. 

Test  Level Test Vehicle Mass Height of Mass Centre
TL-3 and TL-4 Small Car – 820kg 550mm
TL-3 and TL-4 Pickup - 2000kg  700mm
TL-4 Single Unit Truck - 8000kg 1,700mm

In summary:

  • TL-3 and TL-4 provide the same level of crash energy containment
  • TL-4 has the added benefit of limiting the roll-over of higher centre of gravity vehicles
  • Maximum deflection, whether TL-3 or TL-4, to be used for design.

For a copy of NCHRP Report 350 - Recommended Procedures for the Safety Performance Evaluation of Highway Features – click here.


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