Safety Evaluation of Cable Median Barriers in Combination With Rumble Strips on Divided Roads

APPENDIX C. ADDITIONAL INSTALLATION DETAILS FROM MISSOURI

This appendix presents further details about the cable barrier installations from Missouri based on a questionnaire that was sent to the participating States.

Cable Median Barrier and Shoulder Rumble Strip Combination Questions—Missouri

1. What was the “before-period” condition for the treatment sites with respect to rumble strips and cable median barriers?
   • No cable median barriers and no rumble strips.
   • Cable median barriers present but no rumble strips.
   • No cable median barriers but rumble strips present.

     Answer: The median guard cable was not installed in combination with the rumble strips. Both were independent projects, and time of installation will vary. The “before-period” for the locations will not have median guard cable but would have potentially included a 30-inch wide rolled rumble strip (very little value in noise and feel).

2. What type(s) of rumble strips were characteristic of the treatment sites evaluated by this study? (Check all that apply.)
   • Milled.
   • Rolled.
   • Formed.
   • Raised.
   • Other.

     Answer: Missouri uses a milled rumble strip (potential for stamped with concrete pavements). The majority of installed miles to date are milled rumble strips.

3. Can you provide specifications and/or standard drawings that address the following characteristics of the rumble strips evaluated by this study?
   • Width.
   • Length.
   • Depth.
   • Spacing.
   • Lateral placement (i.e., in relation to pavement marking).

     Answer: All of our specifications are in our Engineering Policy Guide (EPG), including standard drawings. MoDOT will install this same rumble strip specification for shoulder widths as low as 2 ft. The link below will provide the policy relating to our rumble strip program:

     http://epg.modot.mo.gov/index.php?title=Category:626_Rumble_Strips

4. Can you provide specifications and/or standard drawings that address the following characteristics of the cable median barriers evaluated by this study?
   • Number of cables.
   • Post spacing.
   • Other important design considerations (i.e., cable pre-stretch, tensioning, slope placement, footing design).

     Answer: Our median guard cable program also has policy provided in our EPG. All current specifications and standard drawings are listed in the policy section. Also, please review the document called MoDOT’s Cable Median Barrier Program, as it provides a lot of good information on our program.

     http://epg.modot.mo.gov/index.php?title=606.2_Guard_Cable

     It is important to note our program began with the low-tension systems, but the majority of miles installed are high-tension.

5. What were the requirements (e.g., minimum paved shoulder width, minimum median width, number of lanes, etc.) for the installation of rumble strips and cable median barriers at the study sites?

   Answer: The cable median barrier program was initially completed on our worst roadways first, and these roads featured very narrow medians (approx. 40 ft) and traffic volumes around 30,000 daily, but we used a systemwide installation method to eliminate the cross-median crash type on the highest need routes. Please see question 4 for more detail, but the treatment has been applied statewide on roadways identified as tier 1 or tier 2.

6. What was the lateral offset from the road to the cable median barriers and how was that distance selected?

   Answer: Historically, our program began with installing the median guard cable in the vertex of the ditch, which was basically in the middle of the median. In 2007, our program changed due to many factors (information on crash dynamics, maintenance issues, and other), and we began installing approximately 8 ft from the stripe. This means it will always be closer to one direction of travel. This information is available in the report identified in question 4 above.

7. Please describe any challenges related to the rumble strip and/or cable median barrier installation and how you overcame them.

   Answer: When MoDOT began the median guard cable program, there was little information available on installation-related issues (location, type, etc.). This ultimately led to a team being formed to determine better policy. I believe the issues with rumble strips relate to both noise and the bicycle community. We initially did not see much pushback from the bicycle community, but we are beginning to see this now as other States have a different specification and standard drawing (more desired by bicycle community). The noise complaints have occurred but have not been a detriment to our program.

8. Please describe any challenges related to the rumble strip and/or cable median barrier maintenance and how you overcame them.

   Answer: With our current specification of the median guard cable location and mow strip, we do not see many maintenance issues (we have an asphalt apron that extends from paved shoulder to approximately 1 ft past cable barrier—this was requested by maintenance). A larger issue related to maintenance of the cable system relates to vehicle crashes. This issue does create a financial issue as our agency is only able to receive about ¹/₃ back in claims compared to impacts with the system.

   The rumble strips have not involved a great deal of maintenance issues due to failures. However, where we have seen areas of failures near the joint, we have allowed sections (not longer than 200 ft) to not be re-milled after a pavement repair. Overall, our system has not seen large-scale failures.

9. What lessons learned or recommendations would you share with another state interested in the widespread application of cable median barriers and rumble strips?

   Answer: The implementation needs to be based on a thorough crash type evaluation and involve widespread installations on a system of routes that would share similar characteristics (regardless of current crash information). In other words, when installing the median guard cable, evaluate characteristics of roads that share the cross-median crash type and install over a network of roadways that are similar (median width and roadway AADT could be your criteria). The same application can be applied on installation of rumble strips on improved shoulders. For instance, do roads with a minimum AADT have an over-representation of the roadway departure crash types? If so, all roads that have this minimum AADT threshold should have a rumble strip installed.

   Also, it is critical to create policy on your safety initiatives to allow your programs to succeed. The policy you develop will drive each program.