6.3.1 Diving Penetrations
Diving-type penetrations were defined as crashes in which the median geometry caused suspension compression, causing the leading edge of the vehicle bumper to dive below the bottom cable and lift all of the cables above the bumper and onto the hood. Diving penetrations were characterized by mechanical levers: once the impacting corner of the vehicle protruded
under the cables, prying action through the longitudinal axis of the vehicle lifted the cables up in the same manner as a long tree limb pulled at a sufficient distance can lift or displace a boulder.
Diving failures were not restricted to particular vehicle classes. Examples of vehicle makes involved in diving crashes included a Saturn Aura, Mitsubishi Galant, Ford Fusion, and Ford Mustang. These vehicles did not conform to any identifiable front-end patterns except that the height of the leading edge of the hood was not “large”, or were all below approximately 27 in. (686 mm) when the vehicle was at rest. Other underride penetration types were heavily dependent on the geometry of the impacting vehicle.
6.3.2 Prying Underride Failures
In prying-type underride penetrations, cables were pried up above the hood, thus allowing the vehicle to pass under the cables. The prying differentiation was used to separate crashes which did not heavily depend on median terrain; any underride containment failure occurring on a flat shoulder adjacent to the travel lane was therefore a prying-type penetration. As a result, crashes with prying underride penetrations had a strong correlation with the vehicle impact orientation angle and vehicle shape.
Prying penetrations were analogous to mechanical wedges, which can split logs when struck with enough force. Analogously, for low-angle prying events, the prying action is similar to a “seesaw”, in which a small child located far from the fulcrum can lift an adult, similar to the way that motion of the rear of the vehicle can cause prying on the front corner or vice-versa. Both diving and prying penetrations shared similar failure mechanisms. However, median geometries and vehicle types varied widely between the two containment failure datasets, prompting researchers to treat each type independently.
Conversely, engagement along the front or rear planar surfaces of the vehicle frequently resulted in penetrations.
During high-orientation angle impact conditions, the vehicle engaged the barrier in a condition which may promote cable separation, lifting, or compression. The increased risk to impacting vehicles was due to a combination of the following factors: (1) vehicle stiffness prevented the cables from creating “furrows” or grooved contact patches on the vehicle, which tended to retain the cables throughout impact; (2) the “approximate equivalent” vehicle profile which came into direct contact with the barrier system changed; and (3) vertical motions of the front or back of the vehicle were exaggerated relative to cable motion at orientation angles approaching 90 or -90 degrees.
6.3.3 Bounce-Over Penetrations
A bounce-over penetration was a specialized crash event in which the impacting vehicle rebounded off a median slope and passed over the top of the barrier system. This type of crash is more common with smaller passenger vehicles since large vehicles more frequently “dig in” to the medians and either roll over or are captured by the barrier.
Bounce-over crashes occur most commonly on medians steeper than 8:1; most bounce- over crashes occurred on medians between 6:1 and 4:1. Medians in this steepness range are often used to facilitate large rain runoff from the road; as a result, many medians have moist or wet soil through much of the year, even in drier climates. The softer median terrain does not facilitate bounce-over for larger vehicles since a large amount of soil is typically displaced after engaging the slope, which dissipates much of the energy contributed to “bouncing”. Smaller vehicles, which frequently have much lower pitch and yaw inertias, bounce due to the impact without displacing much soil. This is why only small to mid-size cars were engaged in bounce-over impacts in this crash database.
6.3.4 Override Penetrations
Simply defined, override penetrations were those in which the vehicle drove over the top of all of the cables before passing to the non-impact side of the cable median barrier. Note that “bounce-over” crashes, in which the impacting vehicle rebounded off of the median slope and passed over the barrier, were segregated from override penetrations due to median slope contributions.
Override crashes could occur due to vehicle profile, vehicle orientation angle at impact, cable entrapment in or on a post, ramp formation, or excessive cable sag. Of these possibilities, cable entrapment, ramp formation, and vehicle orientation at impact were the most common causes. Virtually every large ½-ton or ¾-ton pickup truck class has a rear end bumper height which is approximately 3 to 5 in. (76 to 127 mm) higher than in the front. Since many cable median barrier systems have a top cable mounting height less than 35 in. (889 mm), many rear- leading pickup and large SUV crashes resulted in penetration that likely would have been adequate captured if cable barrier systems were taller. Due to the frequency of penetration crashes by Dodge Ram pickups manufactured in the years between 2002 and 2010 on virtually every cable median barrier system, it is likely that these systems could be at risk of failing to meet crash performance requirements established in MASH.
6.3.5 Low CG Trajectory Angle Penetrations
A broad class of penetrations that spanned multiple penetration mechanisms consisted of low-CG trajectory angle crashes which resulted in penetration. Low-angle impacts leading to barrier penetration occurred on every barrier make. According to the results of NCHRP Report No. 665 [22], approximately 55% of all run-off-road crashes occurred with CG trajectory angles less than or equal to 15 degrees. Due to the difficulty in determining when a penetration crash
occurred, it is likely that the number of low-angle penetrations is underrepresented in the penetration crash database.
Low-CG trajectory angle penetration crashes had contributions from post impacts and cable entrapment on posts, high-susceptibility vehicle front-end profiles and bumper heights, and low energy available for stable crush to occur, in addition to barrier-specific mechanisms. As a result, both override and prying underride penetrations occurred.
The risk of severe crash result associated with this containment failure was dependent on multiple factors. If the low-CG trajectory angle crash resulted in penetration when the barrier was installed on the traffic-side shoulder or approach slope, the vehicle entered the median. Then, the vehicle either increased the CG trajectory angle due to the median slopes and driver reaction, or came to rest in the median. Crashes in which the vehicle came to rest were not severe in general, whereas moderately low-angle cross-median trajectories were frequently severe. When the barrier was installed near the center of the V-ditch, the vehicle always came to a stop in the median. While this generally resulted in a low-severity crash, underride of the Nucor system caused several severe injuries due to occupant compartment deformation from roof crush. Low-CG trajectory angle penetrations did not occur on systems located on the back side of median slopes, but did occur when the barrier was installed on the opposite-side shoulder. Penetrations on the barrier when it was located on the opposite side of the V-ditch frequently resulted in severe crash outcomes.