THE ROLE OF ROADWAY DESIGN

Understanding Guardrails & Edge Drop

Compass Consulting Engineers specializes in evaluating the role of guardrails, pavement edge drop, and other roadway elements as they relate to the reconstruction of a crash.

Assessing Guardrails and Associated Components in a Crash

Guardrails are classified as roadside safety features or appurtenances. Broadly speaking, guardrails are designed to protect and redirect errant vehicles and/or to manage energy when a crash occurs. They are installed when the consequences of striking a guardrail would be less severe than encountering other objects next to the roadway. When a vehicle impacts a guardrail, the guardrail operates to deflect the vehicle back to the roadway, slow the vehicle down to a complete stop, or, in certain circumstances, slow the vehicle down and then let it proceed past the guardrail. Guardrails are a system which must be designed for vehicles that impact at the ends or midspan of the rail system.

Components

The guardrail face is the length of the guardrail extending from the end terminal alongside the road. Its function is always to redirect the vehicle back onto the roadway. The end terminal is the starting point of the guardrail and is commonly designed to manage the energy of an impact. The end terminal functions in two ways. When hit head-on, the impact head slides down the guardrail flattening, or “extruding,” the guardrail and redirecting the guardrail away from the vehicle until the vehicle’s impact energy is dissipated and the vehicle has decelerated to a stop. When hit at an angle, the impact head may partially extrude the guardrail and then "gate" out of the way allowing the vehicle to pass behind the guardrail.

Crashworthiness

Guardrail system performance is assessed through crash tests in a controlled environment. The AASHTO Manual for Assessing Safety Hardware (MASH) presents uniform guidelines for crash testing guardrails and recommends evaluation criteria to assess test results. The vehicle’s size, speed and impact angle are factors that can affect the performance of a guardrail.

How OUR EXPERTS Can Help

HOW OUR EXPERTS CAN HELP

Legal Advocacy: In court, our expert testimony clarifies guardrail effectiveness. Was it a life-saving barrier or a contributing factor?

Accident Reconstruction: We analyze guardrail interactions. Did the rail deflect as designed? Was it properly anchored? Our insights inform collision reconstructions.

Legal Advocacy: In court, our expert testimony clarifies guardrail effectiveness. Was it a life-saving barrier or a contributing factor?

Accident Reconstruction: We analyze guardrail interactions. Did the rail deflect as designed? Was it properly anchored? Our insights inform collision reconstructions.

MITIGATING ROADWAY HAZARDS

Edge Drops in Roadway Design

A pavement edge drop-off occurs when a vehicle unintentionally leaves the travel lane, and the tires drop down onto a lower surface. If there is a vertical discontinuity, or edge drop-off, between the travel lane and the lower surface, the tires contact and “scrub” against the vertical face of the drop-off which prevents the tire from climbing back up to the travel lane until a critical steer input is achieved. Depending on the height and shape of the drop, as well as the vehicle’s speed and angle, this scrubbing can lead to a loss of vehicular control. This phenomenon is well-documented in the literature and well-understood by our engineers.