Rumble strip


Rumble strips are a traffic calming feature to alert inattentive drivers of potential danger, by causing a tactile vibration and audible rumbling transmitted through a vehicle's wheels into its interior. A rumble strip is applied along the direction of travel following an edgeline or centerline, to alert drivers when they drift from their lane. Rumble strips may also be installed in a series across the direction of travel, to warn drivers of a stop or slowdown ahead, or of an approaching danger spot.
In favorable circumstances, rumble strips are effective at reducing accidents due to inattention. The effectiveness of shoulder rumble strips is largely dependent on a wide and stable road shoulder for a recovery, but there are several other less obvious factors that engineers consider during design.

Other names

Rumble strips are also known as audible lines, sleepy bumps, wake up calls, ruffles strips, growlers, drift lines, waker-uppers, and drunk bumps.

Placement

Rumble strips are divided into transverse rumble strips, shoulder rumble strips, and centerline rumble strips, depending on how they are used.
Transverse rumble strips are placed in the travel lanes where most if not all vehicles will cross them. They are used to alert the driver of an upcoming intersection, toll booth or similar hazard. They may cross the entire road from shoulder to shoulder, or they may only be in the wheel paths. Portable rumble strips, also called Andreas strips, can be used to alert traffic to upcoming lane closures or road works to prevent collision with signage and barriers.
Shoulder and centerline rumble strips are used to reduce lane departure crashes. Centerline rumble strips are used on undivided highways to reduce cross-over incidents and resultant head-on collisions. Shoulder rumble strips are used primarily to reduce run-off-road collisions. They alert distracted or drowsy drivers that they are leaving the roadway or crossing the centerline of the road. In this application, they are narrower and outside of the wheelpaths.

Installation

There are several different ways to install rumble strips:
  • Rolled-in, applied to newly laid asphalt pavement while it is still warm and moldable.
  • Milled-in, applied to existing hardened asphalt or concrete roads.
  • Formed, a corrugated form is pressed into fresh concrete.
  • Raised plastic or ceramic units, fastened to asphalt or concrete pavement and often with a reflector built into the edge. Botts' dots are a common installation.
  • Profiled thermoplastic markings are created by fusing thermoplastic to the pavement and create alternating elevation and recession pattern. This can be done as inverted-profile markings or raised-profile markings. Inverted-profile markings are created by pressing a cog rolling over the markings while they are wet to make them corrugated. Raised-profile markings are created by extruding extra thickness of thermoplastic at a specific interval to create bumps. Raised-profile markings are sometimes known as convex traffic lines.
  • "Smart car" virtual shoulder rumble strips, referred to as a lane departure warning system, are increasingly available on personal vehicles and commercial trucks. The alarm is similar to the sound produced when striking rumble strips.
  • "Smart car" virtual transverse rumble strips to prevent cross-path crashes at intersections are being developed.
Surface-mount raised pavement reflectors are easily scraped off by the blade on snowplows, and thus are not practical in many locations in the United States and Canada.
Rumble strips combined with pavement markings are sometimes called rumble stripes. They may be formed with raised textured plastic pavement markers, or they may use conventional pavement marking materials sprayed onto milled rumble strips. Rumble stripes have markedly increased visibility in wet nighttime conditions, when conventional markings on flat surfaces can be difficult to see.

History

An early use of this construction was in 1943 on New Jersey's Route 6 near Great Notch. The scoring in the pavement—at the dual-lane divisions—reflected drivers' headlights, while "the resonant whine or roar coming off the ridges as the vehicle's tires rolled over the strip let the motorist know he was getting out of his traffic lane." With the "singing lane" safety benefit recognized for nearly a decade, the New Jersey chair of its Highway Authority, Ransford J. Abbott, mandated pavement-edge scoring for the Garden State Parkway in the early 1950s.
Rumble strips were first implemented on the Garden State Parkway in New Jersey in 1952.
Initially, shoulder rumble strip installation focused on freeways using rolled-in rumble strips of different designs using a modified roller on a pavement rolling machines. Later, paving contractors modified pavement rolling machines to mill rumble strips into existing hardened asphalt pavement. Specifically designed commercially available machines followed. The development of ceramic and plastic raised systems enabled installation on concrete pavement highways, and the smaller footprint was better suited for the dashed centerline. "Virtual" rumble strips followed.
As rumble strips produce audible rumbling in specific audio frequencies based on the spacing of the grooves and the speed of the automobile, they have been used to create novel musical roads. These are also known as "singing shoulders".
Rumble strip installation is widespread, and in some cases controversial. Residents near urban freeways complain of noise at night as vehicles change lanes; or when vehicles strike the transverse rumble strips. The encroachment of shoulder rumble strips onto highways with narrow shoulders may create a hazard for cyclists. US and Canadian guidelines have minimum standards for installation on known cycling routes. In 2009, in Michigan, the Amish claimed that the shoulder rumble strips were dangerous for horse-drawn carriages, and successfully lobbied to have them paved over. In 2010, Kansas has considered removing shoulder rumble strips from an interstate highway to allow buses to travel on the shoulder during periods of traffic congestion.

Accident and driver dynamics

On-road and run-off-road accidents

Single-vehicle crashes are classified into two groups: run-off-road, and on-road crashes in which the vehicle remains on the road after the crash. ROR crashes can account for up to 70% of the fatal single-vehicle crashes.
ROR crashes are due to inattention, speeding, traction loss, overreaction, crash avoidance, and mechanical failure. Rumble strips only prevent ROR crashes due to inattention.
Research indicates that 47% of RORs exited the highway to the left; while 53% exited the highway to the right.

Inattentive driving

A US Federal Highway Administration sponsored study stated that driver inattention comes in many forms, including distraction, daydreaming/competing thoughts, fatigue/drowsiness, and alcohol/drug impairment. Early evening low alcohol intake also worsens sleepiness-related driving impairment.
In a 2008 survey in the US, 33% of fatally injured drivers tested were found to be legally impaired, and an additional 5% were found to have a legal amount of alcohol in their bodies. Canada has similar statistics.

Migration of accidents

Studies support the hypothesis that some crashes are not prevented, but merely "migrated" or displaced from vehicle to vehicle, season to season, or location to location, and that such crashes may be no less severe than ones prevented by rumble strips.
An FHWA sponsored study wrestled with the moral dilemma of rumble strips keeping "unsafe drivers" on the highway. "This group of unsafe drivers temporarily saved by the rumble strips may have caused some multiple-vehicle crashes involving harm to innocent victims to occur downstream from the treated site where no rumble strips existed. Unfortunately, as noted above, an examination of downstream crashes could not be conducted."
A 2008 Swedish study using a driving simulator and 35 sleep-deprived drivers concluded: "The main results showed an increase in sleepiness indicators from start to before hitting the rumble strip, an alerting effect in most parameters after hitting the strip. The alertness enhancing effect was, however, short and the sleepiness signs returned 5 min after the rumble strip hit. Essentially no effects were seen due to type of strip."
A 2003 Montana study suggested that on Interstates, shoulder rumble reduced the roll-over accident rates but the severity of unprevented roll-over accidents increased. This was thought to be due to the rumble strip "scaring" sleeping drivers to the extent that they overreacted. This problem was more pronounced on primary highways with rumble strips.

"Classic" one-car crashes

The 'classic' one-car crash results when a vehicle slowly drifts to the right, hits dirt or rumble strips on the right shoulder of the road, and the driver becomes alert and overreacts, jerking the wheel left to bring the vehicle back onto the road. This motion causes the left front tire to strike the raised edge of the pavement at a sharp angle, often causing a rollover or a swerve into oncoming traffic. This form of one-car crash is "classic" because it occurs very often. Raised edges of pavement were once common, but are now recognized as a hazard; it is now standard practice to level the gravel shoulder with the pavement, although edge-drops may reform due to soil erosion. This "slowly drift to the right" scenario applies to jurisdictions with right-hand traffic, so in jurisdictions with left-hand traffic it would be a "slowly drift to the left" scenario.
This phenomenon implies that a sleeping driver often does not react and begin to recover, until all four wheels have struck a rumble strip; if the paved shoulder is narrower than the width of the vehicle wheel track, a rumble strip may not prevent a sleeping driver from going off the road.
On a single-lane highway, an overreacting driver has less room to regain control, which may exacerbate their initial overreaction after striking the strips, resulting in a roll-over or head-on collision. A crash investigating officer stated: "It's consistent with someone who falls asleep or overreacts to the rumble strips", which implied that this was not the first time the officer has witnessed this situation. Note that in the KATU.com article photograph of the crash scene, the passenger-side tire print in the soft shoulder that suggests that all four wheels passed over the rumble strip before the driver attempted the unsuccessful recovery.