Project 2: Median Barrier Placement on Six-lane, 46-foot Median Divided Freeways
Sponsor: North Carolina Department of Transportation
Duration: August 16, 2008 – August 15, 2010
Research team: Howie Fang (PI), Ning Li, Ning Tian, Xuchun Ren
Research Overview:
Median-involved crashes on high-speed, divided highways are predominately severe events in terms of injury severity, property damage, traffic impact, and the magnitude and duration of response required (BMI-SG 2004). Median barriers can be used to effectively prevent vehicles leaving the roadway from crossing the median and colliding with vehicles traveling in the opposite direction. Despite the dramatic increase in traffic volumes, the fatal crash rate on U.S. highways is only 20% of what it was 40 years ago. Part of the reason is attributed to the use of roadside barrier systems.
Over the years, different types of barriers have been developed and are classified into three categories: rigid, semi-rigid, and flexible systems. While all barriers serve the purpose of safely redirecting errant vehicles and preventing them from intruding into the oncoming traffic, they differ from each other in applicable site conditions as well as in their effects on impacting vehicles. Figure 1.1 shows several commonly used barrier systems including concrete barriers, W-beam and thrie-beam guardrails, and cable barriers.
Concrete barriers belong to the rigid category and have high initial cost yet require less maintenance; however, they are less forgiving in severe crashes. W-beam, thrie-beam, and modified thrie-beam guardrails are semi-rigid barriers that are more forgiving (or having less impact forces) but have larger deflections than concrete barriers. Cable barriers are flexible systems that are cost-effective and ideally suitable for retrofit designs on existing, relatively wide medians. Cable barriers are more forgiving then concrete barriers and W-beam guardrails, because the cables deflect laterally to absorb energy and reduce impact forces transmitted to the vehicle and occupants. The high flexibility of cable barriers, however, requires the median to have sufficient width to allow for lateral deflections. For a single-run cable median barrier (CMB), the median is required to have a minimum width of 24 ft (7.32 m), with 12 ft (3.66 m) on each side of the barrier (AASHTO 2006).
In this project, full-scale FE simulations were utilized to determine the feasibility of using a single line of double-face W-beam guardrail to replace the two lines of single-face W-beam. FE simulations were also used to evaluate the performance of a generic low-tension CMB placed on six-lane, 46-foot median divided freeways. Vehicle-barrier impacts were simulated under different combinations of impact speeds and angles.
Research Outcome:
High-Side Impact |
Low-Side Impact |
|||
| .GIF |
.AVI |
.GIF |
.AVI |
|
| 25° | 60 MPH | 60 MPH | 60 MPH | 60 MPH |
| 70 MPH | 70 MPH | 70 MPH | 70 MPH | |
| 75 MPH | 75 MPH | |||
| 30° | 60 MPH | 60 MPH | ||
| 70 MPH | 70 MPH | |||
| 75 MPH | 75 MPH | |||
| 35° | 60 MPH | 60 MPH | ||
| 70 MPH | 70 MPH | |||
| 75 MPH | 75 MPH | |||
High-Side Impact |
Low-Side Impact |
|||
| .GIF |
.AVI |
.GIF |
.AVI |
|
| 25° | 60 MPH | 60 MPH | 60 MPH | 60 MPH |
| 70 MPH | 70 MPH | 70 MPH | 70 MPH | |
| 75 MPH | 75 MPH | 75 MPH | 75 MPH | |
| 30° | 60 MPH | 60 MPH | 60 MPH | 60 MPH |
| 70 MPH | 70 MPH | 70 MPH | 70 MPH | |
| 75 MPH | 75 MPH | 75 MPH | 75 MPH | |
| 35° | 60 MPH | 60 MPH | 60 MPH | 60 MPH |
| 70 MPH | 70 MPH | 70 MPH | 70 MPH | |
| 75 MPH | 75 MPH | 75 MPH | 75 MPH | |
