Maricopa County Department of Transportation
Community & Government Relations Division
MEMORANDUM
DATE:August 7, 2002
TO:Mike Sabatini
FROM:Jon White
SUBJECT:AACE Legislative Proposal Re: Speed Limits on Unpaved Roads
Nicolaas Swart provided the following comments. His overall conclusion is in the final paragraph.
I am not quite clear on what the intent of the Amendment is, but here are some of my thoughts.
The Position Paper refers to AASHTO Guidelines on the Geometric Design of Very Low-Volume Local Roads page 50, where the statement is made that "design speeds for unpaved roads should NORMALLY be 45mph." The guidelines also refer to Design Speed/Operating Speed on page 9 and state: "The selected design speed should realistically represent actual or anticipated operating speeds and conditions on the roadway being designed." It then also refers to 0 to 45 mph as low speed and more than 45mph as high speed. It would therefore appear that to design to 45mph would be a goal, but terrain and other condition may determine what the appropriate design speed should be. For example, it would be very difficult to design logging access roads to 45 mph, a road of low volume as referred to in the guidelines on page 50. The reason for this is that to achieve sight distances in a logging environment to comply with 45 mph would be very difficult. Closer to home, we are designing our PM-10 roadways for 35 mph (which is paved), and in some instances, like Spur Cross Road we used a design speed of less than 35mph to fit the mountainous terrain more appropriately. If a design speed of 45 mph was to be used, major earthworks would be required, which would have pushed the construction cost way beyond the intent of dust control and to provide a better road for a low volume situation. The approach to design speed should be based on what is appropriate for the specific situation if a guideline for 45mph seems unrealistic, and the chosen design speed should be used in such a way that a consistent approach is achieve along the entire segment of roadway, in other words the design should provide for a consistent experience to the driver. It would, for example not be appropriate to have a 45mph design for a couple of miles with a 25mph curve in the middle of the segment. So, with all of this said, I think the emphasis should be on reasonable and prudent design speed as compared to try to post a reasonable and prudent speed for a dirt road.
Secondly, design speeds is normally 5 to 10 miles per hour higher than the posted speed to allow for a cushion of additional safety for drivers traveling at speeds higher than the posted. This is even more important on dirt roads as the change in conditions can vary much more and much quicker than that for paved roads. The Position Paper and Amendment 3 suggest that both the design and posted speed should be 45mph. It is also suggested that the proposed 45mph posted speed is based on "those geometrics and surface conditions observed immediately following the maintenance thereof", which is the ideal condition for an unpaved roadway.
In Maricopa County, conditions can change very quickly after storms at washes for example, and to post dirt roads may not be to our best interest. This may not be the case in other Counties.
All the Alternatives refer under E to "A person shall not drive a motor vehicle at a speed that is less than the speed that is reasonable and prudent under existing conditions". If we post a dirt road (Alternative 3) for say 45mph as indicating that it is the prudent speed, could we be in conflict with a condition after a storm where the prudent speed could be much lower?
I agree that the reference to "unpaved roads" in par. 4 under Excessive speeds; Classification, in Alternatives 3 and 4 should read "paved roads".
It is my interpretation of AASHTO, that while it suggests a design speeds of 45mph for unpaved roads, it is just a guideline and that the selected design speed should be reasonable and prudent to suit the specific conditions. The proposed reference under 3 of B of the three different alternatives may therefore not be appropriate. I am not sure that posting dirt roads would be in the best interest for MCDOT. If I have to choose an Alternative, I would go for Alt 2.
Nicolaas