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Could you please provide an interpretation on measuring guardrail deflection?

Based on the AASHTO Roadside Design Guide, the deflection distance is measured from the back of the post. If a roadside object such as light pole is desired to be installed, should the guardrail element width (post + blockout + w-beam) would need to be added to the above-mentioned deflection distance?  Or is the maximum distance the guardrail predicted to deflect should be measured from the back of the w-beam?

Two terms/measurements for deflection, working width and dynamic deflection, have been used on MGS reports. It is wondering if the deflection was conservatively defined as from back of post to object. However, if the posts are intended to fold down on impact, it seems that it's unlikely that they will be the maximum deflected object. The rail would normally be pushed the furthest?

Should the following figure from the AASHTO Roadside Design is the best way to measure deflection, or if deflection should be measured another way?  

The test labs report both dynamic deflection and working width.

The following text is located in MASH with respect to the deflections and working width. They may help serve as basic definitions for you.

“Test article deflections—Report the permanent and dynamic deflections of the test article plus the working width during impact. These measurements normally apply to longitudinal barriers, terminals, crash cushions, and TMAs. Permanent deflection is the residual lateral displacement of the test article remaining after the impact. Dynamic deflection is the maximum lateral displacement of the test article on the traffic side that occurs during the impact. The working width is the maximum dynamic lateral position of any major part of the system or vehicle. These measurements are all relative to the pre-impact traffic face of the test article. For the working width, the height of the maximum working width should also be documented and reported.”

“working width—The distance between the traffic face of the test article before the impact and the maximum lateral position of any major part of the system or vehicle after the impact”

Thus, for dynamic deflection, we are tracking points on the rail and posts in the high-speed film and determining the maximum deflection during the impact from that parts original position.

Working width is the maximum extent of the either the barrier or vehicle past the most forward point of the barrier on the impact face. This a more useful measurement for DOTs as it better defines the design space you are forced to accommodate adjacent to the barrier.

1. Since a rigid barrier does not move and if the vehicle does not extend over it or under it, then the working width is equal to the rigid barrier width from front toe or top face (most forward point) to farthest back side point.
2. If a vehicle extends over the barrier when it is undeformed or deformed, then the working width is equal to the distance between the initial front most forward point on barrier to the farthest backward extent of the vehicle over the deformed barrier.
3. If the vehicle does not extend over the barrier, then the working width is equal to the distance between the initial front most forward point on barrier to the farthest backward extent of the barrier components during the impact event.

Note that working width always uses the most forward point on a barrier. For a single-slope or safety shape barrier, the lower front toe is the most forward point on the barrier. If a barrier has a point near the top that extends farther forward, then that location would be the starting point for which working width is determined.

Working width would define the distance from the face of the rail to the hazard. Neither measurement deals with the distance from the back of the post to the hazard as shown below. However, the working width minus the barrier width would give you the distance from the back of the barrier to the hazard.

Let me know if that helps or if you have any questions.