Rumble Hog Case Studies
January 19, 2022

FHA Addressing Noise Issues On Two-lane Roads

Center line rumble strips or stripes (shown in Figure 1) can prevent head-on collisions and opposite-direction sideswipes, often referred to as cross-center line crashes. They are primarily used to warn drivers who are drifting into an opposing travel lane. While they are most often used on rural two-lane, two-way roadways, they are equally applicable to multilane undivided facilities. Center line markings are typically placed on center rumble strips

Introduction

Longitudinal rumble strips are an effective, low-cost countermeasure for preventing roadway departure crashes, including both run-off-road and opposing-direction crashes. Rumble strips1,2 are milled or raised patterns near the centerline or edge line of the roadway that provides both an audible warning (rumbling sound) and a physical vibration to alert drivers that they are crossing into opposing traffic or leaving the roadway. Rumble strips placed underneath center line or edge line striping are often referred to as “rumble stripes.”

Rumble Strip Basics. There are two main applications for rumble strips

Center line rumble strips or stripes (shown in Figure 1) can prevent head-on collisions and opposite-direction sideswipes, often referred to as cross-center line crashes. They are primarily used to warn drivers who are drifting into an opposing travel lane. While they are most often used on rural two-lane, two-way roadways, they are equally applicable to multilane undivided facilities. Center line markings are typically placed on center rumble strip which have an added benefit of improving the visibility of the marking in wet, dark conditions. Center line rumble strips have been shown to reduce head-on and opposite-direction fatal and injury crashes by 45 percent on rural two-lane roads.

FIGURE 1. Center Line Rumble Strips

FIGURE 2. Shoulder Rumble Strips

Edge line rumble stripes or shoulder rumble strips (shown in Figure 2) are primarily used to prevent run-off-the-road crashes. They warn drivers that their vehicles are drifting onto the roadside. While shoulder and edge line rumble strips perform the same function, their placement sets them apart. Shoulder rumble strips are common on freeways and other divided highways, but the decision to use one or the other on two-lane roads is based on a number of factors. The edge line rumble stripe places the pavement marking within the rumble strip and thereby improves the visibility of the marking in wet, dark conditions. Placing the rumble strip closer to the travel lane also alerts drivers that they are leaving the roadway sooner. Shoulder rumble strips and edge line rumble stripes have been shown to reduce single-vehicle, run-off-road fatal and injury crashes by 36 percent on rural two-lane roads.

Noise Effects of Rumble Strips. Challenges to rumble strip implementation exist in areas where the noise caused by vehicles hitting rumble strips is undesirable to the surrounding environment (e.g., nearby residents, nearby businesses, or in sensitive habitat areas). The sound inside the vehicle is important as it provides a warning to a driver that he or she may be leaving the roadway. However, the sound outside the vehicle may be disruptive to those who live near highways because it is intermittent and differs from other “normalized” sounds in those areas (e.g., highway traffic). While these rumble strips are designed to be traversed infrequently (e.g., when a driver drifts from her lane or makes a passing maneuver), citizen acceptance of State or local agency safety countermeasure strategies should be taken into consideration as it can affect the long-term viability of those strategies. Careful attention to design details can result in less nuisance hits on the rumble strips (i.e., vehicle tires crossing the rumble strip when a crash is not imminent).


How is Rumble Strip Noise Measured?

Transportation agencies use sound-level meters to assess and record noise levels at field locations, as shown in Figure 3. The sound level of rumble strips can be difficult to measure as it occurs more intermittently than normal highway noises. The same basic measurements used to measure highway traffic noise are also used to measure rumble strip noise. The maximum noise level is typically the key consideration for rumble strips, whereas the average noise level, over a period of time, is the key consideration for highway noise. There is currently no method that adequately measures the unique character of rumble strip noise.

Measuring Rumble Strip Noise Inside the Vehicle. Sufficient noise and vibration levels are needed inside a vehicle to alert drivers they are leaving their lane. These critical factors help ascertain the effectiveness of rumble strips and are typically measured using equipment that is placed inside the vehicle near the driver seat.

FIGURE 3. Field Noise Measurements

The Washington State Department of Transportation (WSDOT) indicates that noise issues result from both the volume and the sound frequency level, as low frequency noise travels farther. 6 While the concept of volume is generally understood (more audible sounds typically have a higher decibel level), the frequency isn’t generally considered when agencies take into account the “loudness” of sound. Generally speaking, low-frequency waves will travel farther than high frequency waves because there is less energy transferred to the medium through which the sound passes.7 In the case of rumble strips, the “medium” could mean anything that resides in the physical space (e.g., trees, buildings, etc.) between the rumble strips and the noise-sensitive location (e.g., residential housing, schools, commercial offices). Sound levels decrease by 6 decibels (dB) for a point source and 3 dB for a line source8 every time distance between the noise source and the listener is doubled. For example, when a person shouts, a person standing 6 feet away hears the shout at 74 dB. When increasing the distance between the person shouting and the listener to 12 feet, the listener hears the noise at 68 dB.

Figure 4 shows the variables used to assess highway noise as collected by sound level meters. As noise is measured near rumble strip sites, each variable performs an activity to assess the overall noise level, as described below.

Reducing Noise by Varying Placement

The strategic consideration and placement of center line, edge line, and shoulder rumble strips is important as practitioners try to balance safety improvements with noise concerns. Standard agency practices may take into account noise concerns while recognizing there may be some unique conditions that need to be assessed per individual locations. Some agencies treat rumble strip installation differently depending on roadway geometry, specifically at horizontal curves and where there are passing zones.

For instance, an agency could place shoulder rumble strips farther from the roadway or replace edge line rumble stripes with shoulder rumble strips to reduce the frequency of contact with the rumble strip or stripe. It should be recognized that tradeoffs related to safety, noise, or other concerns such as bicycle accommodation or pavement preservation may occur as a result of any mitigation effort. For this reason, it is important to evaluate the trade-offs associated with a specific