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ODOT's current roadway design guidance from "Location & Design Manual, Volume 1" for using clear zone grading to eliminate the need for barrier protection at a fixed object requires designers to provide clear zone grading for a lateral distance to the back of the hazard and longitudinally along the roadside following the runout length, which varies based on the facility ADT and design speed. The common interpretation amongst district offices is to provide clear zone grading within the limits bound by the Lateral Distance of the Hazard, the Runout Length, and the "Control Line" which creates a triangular area (on page 2 of the attached "Current Guidance.pdf", I hatched the “triangle” area in red).

Proposed guidance modifies the language to require clear zone grading to be provided throughout the “box” that the Lateral Distance of the Hazard and the Runout Length create. In this methodology, the "Control Line” is ignored and the extra area hatched in blue on page 2 of the “Proposed Guidance.pdf” would also need to meet clear zone grading criteria (I attached “Relevant Figures from ODOT Design Manual” that detail ODOT’s clear zone grading criteria for reference). Also, see attached for a sample problem demonstrating the “box” method.

Note that on many of the ODOT projects which employ clear zone grading to eliminate barrier at a culvert, there is not clear zone grading perpetuated for the remainder of the corridor, and the area that is clear zone graded is essentially a spot improvement with transitions to existing conditions on either end. So, defined limits for the clear zone grading must be established by ODOT for designers to follow.

The justification for the “box” is that the control line does not represent a logical boundary when applying clear zone grading, as it is used to intercept the end terminal of guardrail. If an end terminal of a guardrail run is installed so that the LON point is at or past the control line, any motorist departing the roadway along the runout length should theoretically impact the guardrail and be redirected away from the hazard. When applied to clear zone grading, there is no re-directive capacity of the flatter slopes and the control line (in my opinion) is arbitrary. This generates concern that a motorist may not be entirely safe from the hazard (large drop-off at the culvert, stream, headwalls, etc.) if they depart from the roadway near the “begin vertex of the triangle” as the clear zone grading is only provided for a narrow width bound by the control line.

There are criticisms of the proposed guidance, as it is a somewhat significant change from how ODOT has historically handled the clear zone grading for elimination of barrier. There are concerns that the new “box” method will result in increased construction costs, project footprints,  r/w acquisition, utility relocation efforts, etc.

• Is the current “triangle” method an adequate and sufficient application of the clear zone grading concept?  Are there any publications or research that detail the required limits of clear zone grading (as a spot improvement) for elimination of barrier at a fixed object?


• Is the justification for the “box” method sound?


• Do you know how other DOTs generally handle this?

We discussed it internally and have some thoughts/questions.

1. I am not sure I follow what the current ODOT guidance means by “clear zone grading is used to eliminate the need for barrier protection of a fixed object”. Is the fixed object removed and replaced with clear zone guidance? The clear zone is defined as a unobstructed, traversable area. Thus, use of a clear zone would indicate that the hazard has been removed.
2. If you are removing the hazard for the area and installing a traversable clear zone, the use of a box similar to the updated guidance seems to make more sense. The clear zone should be a hazard free area with traversable slopes. As such, those slopes and grading should extend throughout the lateral extent of the clear zone rather that along a line defined by the runout length to a hazard.
3. If the hazard is not removed, then we would generally recommend that a barrier be used to shield the hazard. In that case, the grading in front of and behind the barrier should be based on guidance for the proper function of the barrier. However, the grading behind the barrier LON would not need to comply with traversable clear zone slopes based on existing guidance.

Let me know if that helps. We would be happy to discuss further.

Thanks!

Yes, the fixed object would be removed and replaced to a location outside of the clear zone. Typically, this guidance applies to culvert extensions where the existing culvert end is within the clear zone and shielded by guardrail; the designer extends it beyond the clear zone boundary and provides clear zone grading, which eliminates the need for the barrier. The relocation of the fixed object is assumed, but it does not explicitly state that anywhere so I think we will rewrite that first sentence to provide clarity, something along the lines of: “The need for barrier protection of a fixed object may be eliminated when the fixed object has been relocated beyond the clear zone boundary and clear zone grading has been provided through the required limits on the approach and trailing ends. The clear zone grading shall extend…”

Bullet point #2 answers my question. Thank you for looking into this.