Safety Evaluation of Centerline Plus Shoulder Rumble Strips

Chapter 5. Data Collection

Kentucky, Missouri, and Pennsylvania provided data containing locations and dates of the installation of CLRS and SRS. In Missouri, SRS are placed on the edge line; in Kentucky and Pennsylvania, they are both installed on the edge line and placed further into the shoulder. Throughout the report, the abbreviation SRS will be used to refer to both installation types. These States also provided roadway geometry, traffic volumes, and crash data for both installation and reference sites. This section provides a summary of the data assembled for the analysis.

Kentucky

Installation Data

The Kentucky Transportation Cabinet (KYTC) provided a list of roadway sections where CLRS had been installed, along with an indication of whether SRS or edgeline rumble stripes were installed concurrently. As stated previously, the report will refer to both as SRS in abbreviated form. In cases where the rumble strip is placed on the edge line, these are referred to as rumble stripes because the lane striping is applied on top. Both edgeline and shoulder applications have been applied in Kentucky. The final list of treated sites used for analysis comprises 12 sections (42 mi) where CLRS and SRS were installed at the same time as part of a resurfacing effort, and 15 sections (122 mi) where CLRS had been installed as retrofits. All roadways previously had SRS installed, so the results will strictly pertain to the incremental effects of adding CLRS to roadways that have SRS. Thus, the estimated benefits could be considered as conservative in that even greater crash reductions would be expected for run-off-road crashes if SRS had not previously existed. It should be noted, however, that for the resurfaced sites, it is possible that the rumble strips had exceeded their service life, although this could not be determined.

The two types of treatment groups are described as follows:

• Retrofit treatment sites are sections of road where CLRS were milled into the existing asphalt. These sites were selected as roadways that were wide enough to apply CLRS and had a history of crashes over a certain threshold (3 head-on, sideswipe, or opposite-direction crashes in the 2004-2008 period). The KYTC safety office funded and applied the installations. All sites had a prior condition of rolled SRS.
• Resurfacing treatment sites are sections of road where CLRS and new edgeline rumble stripes were installed simultaneously as part of a resurfacing effort. The KYTC safety office, working in conjunction with the roadway maintenance and pavement office, identified the locations. A contractor performed a field inspection to verify installation date and type. KYTC indicates that these sites were selected as part of the regular resurfacing schedule with no specific consideration of crash history in their selection. Almost all resurfacing sites were rural locations. All sites had a prior condition of SRS or texturing and no CLRS.

Reference Sites

As described above, one set of treated sites was selected for treatment under a retrofit program that installed CLRS on sites that were selected on the basis of high target crashes (i.e., head-on and sideswipe-opposite-direction). To match these retrofit treated sites, the KYTC identified sites that were selected to receive CLRS (with a projected installation date of summer 2012) but had not yet received the treatment. This retrofit reference group comprises 133 mi of road.

The other set of treatment sites was selected as part of the resurfacing effort (and not on the basis of crashes). To match these resurfacing treatment sites, the research team desired to identify a reference group that would match the characteristics of the resurfaced sites and would be eligible to receive resurfacing but would not yet have received resurfacing. The reference group was identified initially by considering all statewide mileage that was undivided (as noted by a median width of 0 ft), had two lanes, had an 11-ft or greater lane width, and had a 50 mi/h or greater speed limit (and was thus eligible to receive CLRS in any resurfacing effort). These locations were cross-referenced against several lists to remove sites that were identified in the State's Roadway Departure plan as having high target crashes (head-on and sideswipe-opposite-direction) and to remove sites that already received CLRS through past construction activities. This resurfacing reference group comprises 1,588 mi of road.

Roadway Data

KYTC staff provided roadway data in geographic information system (GIS) shapefile formats. The various road characteristics (e.g., shoulder width) were contained in separate shapefiles for each segment. GIS files were obtained from the Kentucky Roadway Information and Data Web site. Characteristics of the treatment and reference sites were obtained by matching each study site to the appropriate inventory segment by county, route, and milepost.

Traffic Data

Traffic volume data are maintained by KYTC in the GIS inventory files. Traffic data were obtained for the treatment and reference sites by matching each study site to the appropriate inventory segment by county, route, and milepost. Specifically, the inventory file from year 2010 was used because it provided two data points-a current (2010) AADT and the prior AADT (with an indication of the year taken). These volume points can be used as needed to extrapolate yearly AADT for the before period. Subsequently, a similar file was obtained with 2012 traffic counts. The multiple traffic counts were used to develop annual trends, but these estimates were determined to be unreliable because extrapolating over a significant number of years often resulted in unreasonable values (e.g., negative AADT counts). As such, the average AADT using actual counts was used in both the before and after periods.

Crash Data

KYTC provided crash data for the routes and counties indicated in the treatment and reference site lists for 2002-2012 and a data dictionary for interpreting the fields in the crash data. The crash data can be linked to the sites based on county, route, and begin and end mileposts. The field labeled "RDWYIDTXT" is present in both the crash and road files to indicate the route. Of note, KYTC indicated that crash location quality improved significantly in 2008. This improvement is the result of law enforcement using the Map It application, which the officers could use to select the crash location on a screen, which would apply latitude/longitude coordinates to the crash record.

Treatment Cost Data

KYTC provided estimates of the costs and services lives of the treatments for use in conducting a benefit-cost analysis of the treatment (table 7).

Table 7. Kentucky treatment cost and service life data.

¹Stripes were used in cases when the rumble strip was placed directly on the edge line.
CLRS = Centerline rumble strips.
SRS = Shoulder rumble strips.

Missouri

Installation Data

The Missouri Department of Transportation (MoDOT) provided a list of projects where CLRS and edgeline rumble strips were recently installed or planned to be installed. The total length of roadway with CLRS and SRS installations was 460 mi. Among the data provided by the reports were the location (including district, State route number, and mileposts) and the construction dates. MoDOT reported that some locations also had 6-inch striping, bigger and brighter signs, and delineation on guardrails.

Reference Sites

Missouri now installs rumble strips on two-lane rural roads whenever a resurfacing project is undertaken. As a result, it would be very difficult to identify roadways with no rumble strips for this road type presently or in the near future. For this reason, a separate reference group of sites without rumble strips has not been identified. An alternate approach to the standard EB before-after methodology was applied, which is further described in the section on study design. In short, this method used before period data at the treatment sites to develop SPFs to control for regression-to-the-mean and traffic volume changes. Because the installation of rumble strips is a policy for all resurfacing projects, regression-to-the-mean was not as high a concern as it otherwise may be. Time trends were accounted for using both early installations and later installations.

Roadway Data

MoDOT provided roadway data for the treatment sites and included the following variables:

• Area type (urban/rural).
• Functional class.
• Divided versus undivided.
• Number of lanes.
• Lane width.
• Shoulder type.
• Shoulder width.
• Surface type.
• Speed limit.

The roadway data are stored in a bidirectional manner, meaning there is a separate record for each direction of travel. MoDOT staff matched opposing directions of travel for each site. The constructed database is limited to one record per site and the geometric information taken from the primary direction of travel.

Traffic Data

MoDOT provided traffic data in the form of AADT from 1999 to 2011 in electronic files for all treatment sites.

Crash Data

MoDOT provided crash data from 1999 to 2011, including many variables related to the location, time, and characteristics of each crash.

Treatment Cost Data

MoDOT provided approximate installation costs of $1,000/mi for either edgeline or centerline. MoDOT estimates that the service lives of rumble strips is 7 to 10 year.

Pennsylvania

Installation Data

The Pennsylvania Department of Transportation (PennDOT) provided a list of projects where both CLRS and SRS were recently installed or planned to be installed. The sites used for analysis totaled 218 mi. These data included information on the location (including PennDOT district, county, State route number, and Segment/Offset, which is PennDOT's milepost system) and the project number of the installations. The project number is used for tracking project progress. The project team obtained construction start and end dates from a PennDOT Web site with this number.

PennDOT reported that some locations may have had shoulders widened to accommodate the SRS.

Reference Sites

The project team derived a preliminary list of reference sites by matching PennDOT's rumble strip inventory to the inventory of all rural two-lane roads. Roads having neither CLRS nor SRS were retained for reference sites. PennDOT confirmed that rumble strips have not been applied to any of the reference sites. The list was further reduced by including only those sites whose characteristics matched the range of treatment sites as follows:

• ACC_CODE=3 (no access control).
• Divisor in 0,1,2,3 (none, painted divided, man-made barrier, earth divided).
• DIV_WDT = 0 (divided width equal to 0 ft.).
• Speed Limit 20-55 mi/h.
• Number of lanes = 2.
• AADT between 650 and 26,570.

The sum of reference site miles was 17,931 mi.

Roadway Data

The project team obtained roadway data for the treatment and reference sites from the PennDOT Roadway Management System and included the following variables:

• Surface type.
• Pavement width.
• Speed limit.
• Number of lanes.
• Year of resurfacing.
• Shoulder type.
• Shoulder width.
• Area type (urban/rural).

Traffic Data

The project team obtained traffic data in the form of AADT from PennDOT from 2003 to 2011 in electronic files for all treatment and reference sites. The percentage of trucks in the traffic stream was also provided.

Crash Data

The PennDOT Crash Database is maintained by the Bureau of Highway Safety and Traffic Engineering's Crash Information Systems & Analysis Division. The compiled crash data contain many variables related to the location, time, and characteristics of each crash. Data from 2003 to 2012 were obtained.

Treatment Cost Data

Table 8 provides a breakdown of installation costs/ft provided by PennDOT. This average has steadily decreased over the last decade. In the early 2000s, the average cost was $0.77/ft. Note that these costs are associated with a single "row" or "line" of rumble strips. That cost is for a two-lane roadway with CLRS and SRS; the costs in table 8 should be multiplied by three.

Table 8. Pennsylvania treatment cost data, 2009-2011.

The costs are per ft. Taking into consideration consistent driveway, intersection, and other types of breaks, PennDOT provided an average cost of $1,267/mi for a single line of rumble strips. Therefore, the average cost for CLRS and SRS along a two-lane roadway would be approximately $3,800/mi. These costs assume there are no maintenance costs. PennDOT assumes a life cycle of 7 year for the rumble strips.

Data Characteristics and Summary

Table 9 defines the crash types used by each State. The project team attempted to make the crash type definitions consistent. In all States, intersection-related and animal-related crashes were excluded.

Table 9. Definitions of crash types.

Table 10 provides summary information for the data collected for the treatment sites. The information in table 10 should not be used to make simple before-after comparisons of crashes per mile-year because it does not account for factors, other than the strategy, that may cause a change in safety between the before and after periods. Such comparisons are properly done with the EB analysis as presented later. Table 11 provides summary information for the reference site data. As discussed previously, a different approach was used in Missouri where an appropriate reference group could not be found.

Table 10. Data summary for treatment sites.

AADT = Annual average daily traffic.

Table 11. Data summary for reference sites.

AADT = Average annual daily traffic.