Did You Know: Roadway Design and Guardrails

Posted by on May 10, 2016 in Did You Know | No Comments

Roadway design and guardrailsAs civil engineers and surveyors, there are many aspects of our daily jobs that are second nature to us. But we hear common questions about our field often from the public. In this “Did You Know” blog series, we’ll answer some of these questions.

The Federal Highway Administration’s guidebook entitled Flexibility in Highway Design stresses the importance of understanding that every highway design project is unique. The setting and character of the area, the values of the community, the needs of the highway users, and the challenges and opportunities are unique factors that designers must consider with each highway project.

In this edition of our “Did You Know” series, we will answer common questions that people ask us about roadway design and guardrails.

What steps are involved in starting and completing a road project?

The process of roadway design varies somewhat depending on what agency owns the road and what funding source is paying for it, but these are typical milestones:

1. The project is requested by a sponsor.

2. The project is defined and funding is allocated.

3. Design options for accomplishing the project goals are developed, usually via concept drawings on aerial photography.

4. A field survey is conducted.

5. Preliminary plans are designed, reviewed and approved.

6. The project is bid for construction.

7. Construction begins on the project.

Life cycles for engineering on projects can vary from two to 12 years, depending on funding sources and the size of the project. Some projects are “fast tracked,” depending on the roadway and importance of the project.

Who designs roads and bridges? 

Roads and bridges are typically designed by a project team of professional engineers, project managers and designers.

Are there design standards and guidelines involved with roadway design?

There are many design standards/guidelines involved with roadway design. Several publications provide guidance that has been developed from years of research. The two primary publications engineers rely on are A Policy on Geometric Design of Highway and Streets and Roadside Design Guide, both are published by the American Association of State Highway Transportation Officials (AASHTO).  Most state and local governing agencies have developed their own standards and guidelines based on the aforementioned publications.

What is a guardrail?

A guardrail is a safety barrier intended to stop or re-direct a vehicle that is leaving the roadway in a location where it would be hazardous to run onto the shoulder.  Guardrails are used to shield objects and steep slopes to prevent contact by vehicles.  The size and speed of the vehicle can affect guardrail performance. Generally, a guardrail is defined as a roadside barrier and can come in two forms, flexible and semi-rigid.

A flexible barrier is a type of cable barrier that can absorb a vehicle and does not have a lot of re-directional capabilities. These types of barriers are primarily used in highway medians to prevent roadway head-on collisions.

A semi-rigid barrier is usually constructed of metal, and the impact energy is dissipated through the rail elements, posts, soil and vehicle bodywork.

How are guardrails factored into the design of roadways?

The potential need for guardrails is factored into every roadway design. Their locations are driven by standards previously established for the type of facility, based on design speed, average daily traffic (ADT) and the steepness of the slopes or fixed object they are protecting from errant vehicles.

Who installs guardrails? 

Guardrails are typically installed by an approved contractor based on a set of plans, standard drawings and specifications. All contractors have to be pre-approved and on file by the governing agency before they can install anything on a project.

Are there overall requirements/sizes for guardrails?

There are many overall requirements for the size, height, spacing, etc. of guardrails. These requirements are typically defined in AASHTO’s Roadside Design Guide. Legislative restrictions can vary, but generally state and local governing agencies follow the aforementioned document for guidance on roadway design and guardrail placement, because so much testing and research has been compiled over the years.

As you can see, there are many aspects to be considered when thinking about roadway design and the placement of guardrails. To read more in our “Did You Know” blog series, click here.


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