Question

Identify which of the following items we need to know in order to establish a clear zone value:
          Design Year Traffic (ADT)
          Design Speed
          If the section of road is curved or not curved and the radius size if curved.
          Ground conditions outside of our EOTW ( such as shoulders and ditch slopes)

Answer 1

Definition of Clear Zone:

                The total roadside border area, starting at the edge of the traveled way that is available for use by vehicles. The width of clear zone is depend upon traffic volumes, speeds, roadside geometry.

                The current edition of the AASHTO Roadside Design Guide presents information on the latest state-of-the-practice in roadside safety.

                To find the clear zone value:

1. Obtain the design speed
2. Select the minimum recoverable terrain from AASHTO TABLES based on lane type and design speed.
3. Select the horizontal clearance requirements

Note:

                Recoverable when it is safely traversable and on a slope that is 1V:4H or flatter.

Some examples of roadside elements requiring horizontal clearance:

                Curbs, walls, barriers, piers, signs signal supports, power poles are primary examples of the type of features that can effect a driver’s speed or lane position if located too close to the roadway edge.

The clear zone needs to be maintained behind a curb:

                The difference between a clear zone & a horizontal clearance has been a topic of much confusion. When the green Book & the roadside Design guide were last updated, the AASHTO committees coordinated to dispel the misunderstanding that 2’ behind a curb constituted a clear zone. Since curbs are now generally recognized as having no significant containment or redirection capability, clear zone should be based on traffic volumes and speeds, both without or with a curb.