Safety Evaluation of Edge-Line Rumble Stripes on Rural Two-Lane Horizontal Curves

Chapter 1. Introduction

This chapter presents background information on the strategy of using edge-line rumble stripes (ELRSs), the goals of the study reported here, and a review of the existing literature on the use of rumble strips.

Background on Strategy

ELRSs are a variation of the common shoulder rumble strips (SRSs) used to alert drowsy or distracted drivers when they leave the travel lane to the right. SRSs and ELRSs both target run-off-road (ROR) crashes. Key distinctions between SRSs and ELRSs are provided in the following points:

• SRSs are provided on the shoulder between the pavement marking and the outside edge of the pavement and can be defined by their offset from the edge-line pavement marking.
• ELRSs are installed where the edge-line pavement marking would normally be placed, and the pavement marking is installed directly over the rumble strip. In this way, the rumble stripes are installed closer to the travel lane than common SRSs. In addition, vertical faces are created within the milled rumble strip to which pavement markings are applied, thereby enhancing the visibility of the edge line during nighttime and wet-weather conditions.

Application of ELRSs varies among States depending on climate and roadway surface type. In colder areas, rumble strips are milled into the surface of the roadway, allowing them to be snowplowable. In areas that do not receive snowfall, profiled thermoplastic pavement markings can be used. In addition, profiled thermoplastic pavement markings have been used as an alternative to milled ELRSs for roadways with a chip seal surface. The research in this study focused on the safety effectiveness of milled ELRSs.

Several research studies have examined the use of SRSs; however, research into the performance of ELRSs has been rare and has not been rigorously evaluated. In addition, milled rumble strips have been installed on roadway segments consisting of both horizontal tangents and horizontal curves. Installations on only horizontal curves have been uncommon, and therefore, safety effectiveness evaluations have not focused on their effectiveness on horizontal curves specifically. This study focused on the safety effectiveness of ELRSs on rural two-lane horizontal curves taken from rumble strip installations that were not specific to horizontal curves.

Additional details concerning current practice with rumble strips can be found on the Federal Highway Administration (FHWA) Rumble Strip Community of Practice Web page.⁽¹⁾ This site provides a description of the three major types of rumble strips (milled, rolled (or formed), and raised), detailed construction drawings, effectiveness estimates, interviews with users and other experts, and other important material.

Background on Study

In 1997, the American Association of State Highway and Transportation Officials (AASHTO) Standing Committee on Highway Traffic Safety, with the assistance of FHWA, the National Highway Traffic Safety Administration, and the Transportation Research Board Committee on Transportation Safety Management, met with safety experts in the field of driver, vehicle, and highway issues from various organizations to develop a strategic plan for highway safety. These participants developed 22 key emphasis areas that affect highway safety. The 22 emphasis areas were published in the AASHTO Strategic Highway Safety Plan.⁽²⁾

The National Cooperative Highway Research Program (NCHRP) then published a series of guides to advance the implementation of countermeasures targeted to reduce crashes and injuries. Each guide addresses one of the key emphasis areas and includes an introduction to the problem, a list of objectives for improving safety, and strategies for each objective. Each strategy is designated as proven, tried, or experimental. Many of the strategies discussed in these guides have not been rigorously evaluated; about 80 percent of the strategies are considered tried or experimental.

In 2005, to support the implementation of the guides, FHWA organized a pooled fund study (PFS) to evaluate low-cost safety strategies as part of this strategic highway safety effort. Over time, the pooled fund has grown in size and, at the time of this study, included 40 States. The PFS evaluates the safety effectiveness of several tried and experimental, low-cost safety strategies through scientifically rigorous crash-based studies. FHWA selected the use of ELRSs as a strategy to be evaluated as part of this effort.

Literature Review

The Manual on Uniform Traffic Control Devices (MUTCD) defines rumble strips as either slightly raised or depressed road surfaces with a rough texture designed to provide a haptic alert for inattentive drivers leaving the travel lane.⁽³⁾ As shown in figure 1, rumble strips may be installed either on the shoulders or center line on rural two-lane highways, and the edge line may be adjacent to the rumble strip or overlapping with it, creating a rumble stripe.

 

Figure 2 provides an illustration of rumble strip dimensions, which are explained as follows:⁽⁴⁾

• A—Offset: Distance from the pavement marking (delineating the edge of the traveled way) to the inside edge of the rumble strip.
• B—Length: Dimension of the strip that is perpendicular to the travel directions of the roadway. This is often referred to as the transverse width of the rumble strip.
• C—Width: Dimension of the strip that is parallel to the travel direction of the roadway.
• D—Depth: Maximum distance from the surface of the roadway to the bottom of the rumble strip.
• E—Spacing: Distance between adjacent rumble strips. It is most often measured from the center of the strip to the center of the adjacent strip.
• F—Gap: Distance from the edge of the rumble strip to edge of rumble strip when there is a break in the pattern. Gaps are commonly used to allow bicycles to cross the rumble strip pattern, to allow passing vehicles to cross center-line rumble strips (CLRSs), and to allow for turning movements at intersections and driveways.

 

The University Transportation Center for Alabama conducted an evaluation of ELRS markings in terms of service life, lifecycle cost, and wet and dry visibility in comparison with flat thermoplastic edge markings (FTMs).⁽⁵⁾ The authors measured nighttime dry and wet retroreflectivity at 16 1-mi FTM segments and 5 2-mi ELRS segments. They found that initial dry retroreflectivity was similar between the two groups; however, ELRSs lost visibility at a lower rate due to cumulative traffic passes. ELRSs were found to provide a higher dry retroreflectivity and longer service life than FTMs under similar traffic conditions. For wet retroreflectivity, the ELRSs had a higher initial value than FTMs, but the degradation could not be compared between the two because of a lack of data for FTMs. The cost per mile for ELRSs was $2,424 for a 5-year marking service life and an 8-year lifecycle.

A study by Miles et al. used video data to examine the impacts of CLRSs and ELRSs on passing operations and lateral position on Texas highways.⁽⁶⁾ After application of milled CLRSs on no-passing and passing zones, the authors found no change in passing opportunities or the percentage of vehicles that passed. However, center-line crossing time increased significantly, and gap distance decreased significantly, irrespective of the speed of the data-recording vehicle. For lateral position, vehicle placement shifted farther from the center line after implementation of CLRSs. After implementation of ELRSs, researchers noted a decrease of about 50 percent in shoulder encroachments as well as a significant reduction in other encroachments, including inadvertent contact with the edge line.

Carlson et al. examined wet-night visibility of pavement markings using experimental drivers on a closed rain tunnel.^(7,8) The study tested nine different treatments in random orders and measured perception distance for each sample location. The driver alerted the researcher when he or she observed a marking and when the type could be determined. This research included testing rumble stripes. The findings suggested there was little difference in detection distance between flat thermoplastic lines and rumble strip lines at low rainfall rates. However, the detection distance was 13 to 38 percent greater for rumble strip lines for medium and heavy rainfall rates.

The Mississippi Department of Transportation installed ELRSs on a portion of Interstate 59 with generally encouraging results.⁽⁹⁾ Preliminary data indicated that the strategy provided an excellent audible alert, increased visual awareness of the travel lane, increased reflectivity, and provided results similar to those for inverted profile striping, and Mississippi residents welcomed the installation. However, some concerns associated with the rumble stripes arose, including noise pollution and the potential for an increase in over-correcting and head-on crashes.

Torbic et al. summarized numerous studies on SRSs.⁽¹⁰⁾Table 1 summarizes their results of many States’ studies (negative percentages indicate a decrease in crashes), and their NCHRP report outlines several key findings.⁽¹⁰⁾ The report notes that SRSs installed along freeways made up the majority of the safety effectiveness evaluations and that only a small percentage of the studies evaluated the safety effectiveness of nonfreeway installations. While the evaluations generally focused on crash types most directly affected by rumble strip presence, such as single-vehicle ROR (SVROR) crashes, several studies looked at the safety effect on total crashes. Rumble strip application showed an average reduction of 36 percent in SVROR-type crashes, with a range of 10 to 80 percent. The reduction of total crashes ranged from 13 to 33 percent, with an average reduction of 21 percent.⁽¹⁰⁾

Table 1. Summary of studies on changes in target collision frequency from application of SRSs.⁽¹⁰⁾

State	Type of Facility	Type of Collision Targeted	Percent Change in Target Collision Frequency (Standard Deviation)	Type of Analysis
Arizona(11)	Interstate	SVROR	−80	Cross-sectional comparison
California(12)	Interstate	SVROR	−49	Before–after with comparison sites
California(12)	Interstate	Total	−19	Before–after with comparison sites
Connecticut(13)	Limited-access roadways	SVROR	−32	Before–after with comparison sites
Florida(11)	—	Fixed object	−41	Naive before–after
Florida(11)	—	Ran-into-water	−31	Naive before–after
Illinois and California(14)	Freeways	SVROR (total)	−18 (± 6.8)	Before–after with marked comparison sites and a comparison group
Illinois and California(14)	Freeways	SVROR (injury)	−13 (±11.7)	Before–after with marked comparison sites and a comparison group
Illinois and California(14)	Rural freeways	SVROR (total)	−21.1 (±10.2)	Before–after with marked comparison sites and a comparison group
Illinois and California(14)	Rural freeways	SVROR (injury)	−7.3 (±15.5)	Before–after with marked comparison sites and a comparison group
Kansas (unpublished, cited in Stutts(15))	Freeways	SVROR	−34	Unknown
Maine (16)	Rural freeways	Total	Inconclusive	Before–after with comparison sites
Massachusetts (unpublished, cited in Stutts(15))	—	SVROR	−42	Unknown
Michigan(17)	—	SVROR	−39	Cross-sectional comparison
Minnesota(18)	Rural multilane divided highways	Total	−16	Naive before–after
Minnesota(18)	Rural multilane divided highways	Injury	−17	Naive before–after
Minnesota(18)	Rural multilane divided highways	SVROR (total)	−10	Naive before–after
Minnesota(18)	Rural multilane divided highways	SVROR (injury)	−22	Naive before–after
Minnesota(18)	Rural multilane divided highways	Total	−21	Before–after with comparison sites
Minnesota(18)	Rural multilane divided highways	Injury	−26	Before–after with comparison sites
Minnesota(18)	Rural multilane divided highways	SVROR (total)	−22	Before–after with comparison sites
Minnesota(18)	Rural multilane divided highways	SVROR (injury)	−51	Before–after with comparison sites
Minnesota(19)	Rural two-lane roads	SVROR (total)	−13 (8)	Before–after EB analysis with reference group
Minnesota(19)	Rural two-lane roads	SVROR (injury)	−18 (12)	Before–after EB analysis with reference group
Montana(20)	Interstate and primary highways	SVROR	−14	Before–after with comparison sites
New Jersey (unpublished, cited in Stutts(15))	—	SVROR	−34	Unknown
New York(21)	Interstate parkway	SVROR	−65 to 70	Naive before–after
Pennsylvania(22)	Interstate	SVROR	−60	Naive before–after
Tennessee(23)	Interstate	SVROR	−31	Unknown
Utah(24)	Interstate	SVROR	−27	Before–after with comparison sites
Utah(24)	Interstate	Total	−33	Before–after with comparison sites
Virginia(25)	Rural freeways	SVROR	−52	Before–after with comparison sites
Washington(26)	—	Total	−18	Naive before–after
Multistate(11)	Rural freeways	SVROR	−20	Before–after with comparison sites

Note: This table is adapted from table 4 in Torbic et al. (2009).⁽¹⁰⁾
—Information was not available.

 

Of all the rumble strip crash reduction studies reviewed by Torbic et al., only one (Patel et al.) specifically addresses rural two-lane roads.⁽¹⁹⁾ That study focused on roads in Minnesota and used the EB analysis approach, which is generally more accurate than alternative before–after analysis types. Therefore, from among the listed sources, the Minnesota study appeared to provide the most relevant and reliable indications of the potential safety effects of ELRSs. It estimated a crash reduction of 13 percent (standard error (SE) = 8) for all SVROR crashes and 18 percent (SE = 12) for SVROR injury crashes. It should be noted, however, that these crash reduction factors only applied to rural two-lane roads with an AADT greater than 4,000.⁽²⁷⁾

Torbic et al. examined the safety effectiveness of SRSs on rural two-lane highways.⁽¹⁰⁾ The EB before–after results indicated no change in crashes after application of SRSs for total crashes and fatal and injury crashes for combined data from Minnesota, Missouri, and Pennsylvania. The results indicated a significant 16-percent decrease in SVROR crashes and a significant 36‑percent decrease in SVROR fatal and injury crashes at combined sites. Additional analyses indicated that Pennsylvania had a significant 24-percent reduction in total crashes, 44‑percent decrease in SVROR crashes, and 37-percent decrease in SVROR fatal and injury crashes. In consideration of all analytical methods employed, Torbic et al. recommended the following CMFs for SRSs on rural two-lane roads based on their research:⁽¹⁰⁾

• 0.84 for SVROR crashes.
• 0.64 for SVROR fatal and injury crashes.

In addition, Torbic et al. quantified the impact of SRS placement on safety, focusing on SVROR fatal and injury crashes. Placement was defined as edge line and non-edge line, which were compared with no rumble strips. ELRSs were defined as rumble strips with an offset distance of 0 to 8 inches, and non-ELRSs were defined as having an offset of 9 inches or more. For two-lane rural roadways, there was no significant or practical difference between ELRSs and non-ELRSs. Also, there was no evidence that suggested SRSs resulted in a reduction of SVROR crashes involving heavy vehicles.⁽¹⁰⁾

Khan et al. evaluated the safety benefits of SRSs on rural two-lane highways in Idaho.⁽²⁸⁾ The authors conducted an EB before–after analysis using data from 178.63 mi of data from treatment sites. The results indicated a 14-percent reduction in ROR crashes. Further analysis indicated a 33-percent reduction in ROR crashes for sections with an AADT less than 1,000. In addition, SRSs were most effective on horizontal tangents and horizontal curves with moderate curvature. The study found that SRSs were most effective for paved shoulder widths of 3 ft or more.⁽²⁸⁾

Potts et al. evaluated the safety impacts of wider pavement markings with both CLRSs and ELRSs with resurfacing on rural two-lane highways in Missouri.⁽²⁹⁾ The EB analysis indicated a significant 47.4-percent reduction in fatal and disabling injury crashes and a significant 38.3‑percent reduction in fatal and all injury crashes. A benefit–cost (B/C) evaluation indicated a B/C ratio of 35.6 for wide markings and both CLRSs and ELRSs with resurfacing on rural two-lane roadways.⁽²⁹⁾

Lyon et al. evaluated the safety impacts of combined SRSs and CLRSs using data from Kentucky, Missouri, and Pennsylvania.⁽³⁰⁾ Kentucky data included SRSs and ELRSs, and the final data included sites where SRSs/ELRSs and CLRSs were installed concurrently as part of a resurfacing effort or where CLRSs had been installed as retrofits. Table 2 provides the dimensions of the rumble strips implemented in each of the three States. Note that Pennsylvania had two typical applications for CLRSs and an alternative design for bicycle-tolerable rumble strips.

Table 2. Rumble strip dimensions from Lyon et al.⁽³⁰⁾

Location	Type	Width (inches)	Length (inches)	Depth (inches)	Spacing (inches)
Kentucky	CLRS	7–7.5	12	1/2–5/8	24
Kentucky	SRS	7 ± 1/2	16	1/2 ± 1/8	12 ± 1
Missouri	CLRS	7 ± 1/2	12	7/16 ± 1/16	12 and 24
Missouri	SRS	7 ± 1/2	12	7/16 ± 1/16	12
Pennsylvania	CLRS 1	7 ± 1/2	16	1/2 ± 1/16	24 and 48
Pennsylvania	CLRS 2	7 ± 1/2	14–18	1/2 ± 1/16	24
Pennsylvania	ELRS	5 ± 1/2	6	1/2 ± 1/16	7
Pennsylvania	Bike-tolerable SRS1	5 ± 1/2	16	3/8 ± 1/16	7
Pennsylvania	Bike-tolerable SRS2	5 ± 1/2	16	3/8 ± 1/16	6

¹Roadway’s posted speed limit was greater than or equal to 55 mi/h.
²Roadway’s posted speed limit was less than 55 mi/h.

 

The EB analysis indicated the following significant CMFs for combined States:⁽³⁰⁾

• 0.80 for total crashes (excluding intersection-related and animal crashes).
• 0.77 for fatal and injury crashes.
• 0.74 for ROR crashes.
• 0.63 for head-on crashes.
• 0.77 for sideswipe-opposite-direction crashes.
• 0.70 for head-on and sideswipe-opposite-direction crashes.
• 0.73 for ROR, head-on, and sideswipe-opposite-direction crashes.

Further disaggregate analyses by Lyon et al. indicated significant reductions in Kentucky and Missouri but not in Pennsylvania.⁽³⁰⁾ The authors surmised that earlier installations (which were used by Torbic et al.) were higher-crash locations, while more recently treated sites did not have a high target crash issue (and therefore no safety benefit).⁽¹⁰⁾ Additional analysis by Lyon et al. indicated the following:⁽³⁰⁾

• Larger reductions in ROR crashes for higher traffic volumes (greater than 3,200 AADT).
• Larger reductions in head-on and sideswipe-opposite-direction crashes for lower traffic volumes (less than 9,200 AADT).

A B/C analysis found an estimated B/C ratio between 20.2 and 54.7 based on estimated service lives of 7 to 12 years and estimated annual costs of $557 to $1,511/mi.⁽³⁰⁾

Sayed et al. evaluated the safety effectiveness of CLRSs and SRSs alone and combined on rural two-lane and four-lane divided highways in British Columbia using an EB before–after study design.⁽³¹⁾ The combined application on rural two-lane highways resulted in a 21.4-percent reduction in off-road right, off-road left, and head-on collisions combined. For rural two-lane highways, SRS applications resulted in a 26.1-percent reduction in off-road right collisions, and CLRS applications resulted in a 29.3-percent reduction in off-road left and head-on collisions.⁽³¹⁾

Torbic et al. evaluated the effect of combined CLRSs and SRSs using data from approximately 80 mi of treated roadways in Mississippi.⁽³²⁾ The target crash types evaluated included SVROR crashes left or right, sideswipe-opposite-direction crashes, and head-on crashes. Crash severities evaluated individually included total crashes, fatal and injury crashes, and fatal and serious injury crashes. The results of the EB before–after analysis indicated a significant 35-percent reduction in total target crashes, significant 40-percent reduction in fatal and injury target crashes, and an insignificant 12-percent increase in fatal and serious injury target crashes.⁽³²⁾

Kay et al. evaluated the safety impacts of CLRSs and combined CLRSs and SRSs on rural two-lane highways in Michigan.⁽³³⁾ The EB before–after analysis examined approximately 3,000 mi of CLRS applications and 1,075 mi of combined CLRS and ELRS applications. The results for CLRSs indicated the following significant reductions (K, A, B, C, and O refer to the KABCO scale used to represent injury severity in crash reporting where K is fatal injury, A is incapacitating injury, B is non-incapacitating injury, C is possible injury, and O is property damage only):⁽³³⁾

• 15.8 percent for total crashes.
• 27.3 percent for target crashes.
• 52.9 percent for target-wet pavement crashes.
• 1.4 percent for target-wintry pavement crashes.
• 42.8 percent for target-passing crashes.
• 28.8 percent for target-impaired driving crashes.
• 44.2 percent for target-K injury crashes.
• 32.0 percent for target-A injury crashes.
• 39.3 percent for target-B injury crashes.
• 27.9 percent for target-C injury crashes.
• 16.2 percent for target-O crashes.

The results for combined CLRSs and SRSs indicated the following significant reductions:⁽³³⁾

• 17.2 percent for total crashes.
• 32.8 percent for target crashes.
• 55.6 percent for target-wet pavement crashes.
• 4.6 percent for target-wintry pavement crashes.
• 35.7 percent for target-passing crashes.
• 39.9 percent for target-impaired driving crashes.
• 51.4 percent for target-K injury crashes.
• 32.5 percent for target-A injury crashes.
• 53.7 percent for target-B injury crashes.
• 35.2 percent for target-C injury crashes.
• 28.5 percent for target-O crashes.

Target crashes were identified manually as crashes involving a vehicle crossing the center line of the roadway.

Olson et al. conducted a before–after evaluation of combined CLRSs and SRSs on rural two-lane highways in Washington.⁽³⁴⁾ The analyses compared simultaneous installations, installations where CLRSs were later added to sections with SRSs, and installations where SRSs were later added to sections with CLRSs. In addition, the authors analyzed composite conditions where there were no rumble strips in the before period and conditions with both CLRSs and SRSs, disregarding when they were installed.

For simultaneous installations, the application resulted in a 63.3-percent reduction in lane departure crashes, a 65.4-percent reduction in crossover crashes, and a 61.4-percent reduction in ROR right crashes. Installations were noted to be more effective at higher speeds and for sections with shoulders greater than 4 ft.⁽³⁴⁾

For sections where CLRSs were added to SRSs, the application resulted in a 64.7-percent reduction in crossover crashes and an 8.5-percent increase in ROR right crashes, resulting in a combined 44.6-percent reduction in lane-departure crashes. For sections where SRSs were added to CLRSs, the application resulted in a 47-percent reduction in ROR right crashes and a 6.8‑percent reduction in crossover crashes, resulting in a 37.2-percent reduction in lane-departure crashes.⁽³⁴⁾

The composite analysis indicated a 66-percent reduction in lane-departure crashes and a 56‑percent reduction in fatal and serious injury crashes. The combined application was noted to be slightly more effective for 11-ft lane widths than 12-ft lane widths.

Kubas et al. evaluated the safety effectiveness of CLRSs and SRSs and SRSs only on rural two‑lane highways in North Dakota.⁽³⁵⁾ The authors compared before- and after-crash rates to estimate the effectiveness of rumble strip applications for various crash types. The installation of CLRSs and SRSs resulted in a 2-percent decrease in total crashes, 45-percent decrease in fatal crashes, 21-percent increase in injury crashes, 5-percent decrease in property damage only crashes, and 29-percent decrease in ROR crashes based on a limited sample. The installation of SRSs resulted in a 15-percent decrease in total crashes, 22-percent decrease in property damage only crashes, and 97-percent increase in ROR crashes based on a limited sample. It should be noted that no CMFs from this study received more than a two-star rating in the CMF Clearinghouse.⁽³⁵⁾