Impact of Changing to a 16 Ball Bank Reading for Curve Speed Advisories
Research Problem Statement
It has long been argued that posted speed advisories for many horizontal curves are too low relative to actual travel speeds and safe conditions at the curves. The prevailing assumption among motorists and many professionals is that most curves can be safely and comfortably traversed at speeds of 5, 10 or more mph over the posted advisory. The conventional wisdom notwithstanding, some curves are posted at arguably more realistic speeds and some may even be too high. This results, in part, from different jurisdictions use different methods and instruments to determine the appropriate advisory speeds. In the 2003 Manual of Uniform Traffic Control Devices, section 2C.46 provides the option of using a 16° ball bank indicator reading to set the advisory speed, which is stated to approximate the 85th percentile speed. Widespread adoption of a uniform rule with 16° as the speed criterion would result in more uniform determinations of speed advisories and more consistent signing.
While a consistently-applied uniform rule should have positive results over the long term, it is not clear how motorists will respond to what might be locally significant changes in the posted advisory speeds in the near term. This is especially true if change is implemented by some agencies but not others. Regardless of the basis for the posted advisory, some motorists may tend to always exceed it by the same increment as is currently observed. While greater consistency in signing would be achieved over the long term, the change-over period might last several years and actually result in greater inconsistency and potential motorist uncertainty of the appropriate speed during that period. Finally, if motorists do not respond appropriately to higher advisory speeds, there could also be a safety cost. Thus, it is not clear that potential benefits of change would outweigh potential costs during the transition period.
The overall objectives of this research are to assess the impacts of changing to a 16° uniform rule for determining the appropriate advisory speeds on horizontal curves and recommend whether such a rule should be adopted. More specific objectives include:
1. Determine a) the current state of the practice among state highway/transportation departments and other local agencies regarding methods for determining speed advisories and b) whether current methods will likely be changed in light of the language in Section 2C.46 of the MUTCD.
2. Compare the outcomes of various methods for determining appropriate speed advisories with a 16° rule in order to assess the potential magnitude of the changes in posted speed advisories as a function of the curve geometry and the current practice.
3. Undertake field tests to determine immediate, short-term, and longer-term motorist reactions to changes in posted speed advisories resulting from a change to the 16° rule.
4. Estimate costs and benefits of widespread adoption of the 16° rule.
5. Develop a recommendation regarding the impact and/or further emphasis of the 16° rule in, and including potential changes to, the MUTCD and/or other manuals and references (e.g., materials for inclusion in the TCD Handbook).
horizontal curves, advisory speed, ball bank indicator, 85th percentile speed
Numerous researchers have addressed the issue of appropriate speed advisories for horizontal curves over the years and much of this work was summarized by Chowdhury et al. (ITE Journal v68 n2, 1998)where the use of the 16° rule was endorsed. Some preliminary work on the state of the practice was also included in the final report from NCHRP Project 3-61, Communicating Changes in Horizontal Alignment. There have also been other reviews on this topic including a Texas Transportation Institute survey. The Federal Highway Administration also has a related project pending. What is not contained in the literature is documentation of the effects of actually making potentially wholesale changes in advisory speeds based on a new rule.
Estimate of Problem Funding and Research Period
In order to achieve the research objectives of assessing the immediate, short-term, and long-term impacts of a rule change, a 2-3 year project is necessary so that effects of the advisory speed changes can be appropriately assessed while controlling for the novelty effects of change and other shorter-term perturbations. It would also seem that data collection would have to occur in a number of jurisdictions in different states/regions. It is estimated that $500,000 over 3 years would be required to achieve the objectives.
Urgency, Payoff Potential, and Implementation
As noted above, the use of the 16° ball bank indicator reading was officially noted in the 2003 MUTCD as an option. Some jurisdictions have interpreted this to mean that it should be used, while others are now considering whether to officially endorse the method. In either case, use/adoption of the method is an immediate concern and, thus, the project should have a high priority. The payoff is, likewise, immediate for agencies charged with signing and marking state and local roads. The results of the research would be implemented through changes to the MUTCD and other related manuals and references.