Rumble strips whether used on
edges or centerline of roads are considered
a road safety feature as the vibration and rumbling that they produce help to
alert drivers who are inattentive.
Rumble strips and centerline rumble strips are being used to reduce
accidents and warn inattentive drivers. Rumble strips can even be laid across a
travel lane so that they can warn drivers that they are approaching areas ahead
where they need to exercise caution. Such strips are also accompanied by
signage warning of the impending danger. Rumble strips were first used in 1952
and have followed a number of designs. Initially the asphalt pavement was
itself milled or formed in such a way as to create the indentations required
for the rumble strips. This has now been totally replaced by ceramic or plastic
raised systems made famous by Botts’ Dots. Such strips produce rumbling that
creates specific frequencies in the audible range and traffic engineers have
used this to create singing shoulders or musical roads.
Rumble strips have however not
been without their share of controversy, and people staying near freeways have
complained against the noise produced by such rumbling strips. As a result many
authorities do not install rumble strips in suburban areas where low speed
restrictions are in place. Rumble strips are also sometimes viewed as a hazard
for cyclists, whose narrow wheel base may find the rumble strips difficult to
ride over. Climate is another factor that does contribute to the success or
failure of a rumble strip. In northern areas prone to severe winters, such
strips can get filled with ice or even traction sand and become ineffective. This
can also happen in desert or sandy areas where winds can fill up the space in
the rumble strips. Steering wheel vibration is another aspect that worries
traffic engineers, and this has something to do with the gaps between rumble
strips and is being studied for the best pattern to be followed.
Centerline rumble strips are mainly used on
two lanes, two way roadways and are meant to create warnings for drivers to
avoid potential accidents with opposing traffic. Accidents happen due to side swiping or head
on collisions. It has been established that the installation of centerline rumble strips has led to reduction in such
accidents, especially on rural roads, most of which fall into the two lanes,
two way roads category. Such roads do not have any dividing medians and thus no
impending mechanisms to traffic running in opposite directions.
Different designs of the centerline rumble strips have led to the use of double
4 inch strips laid parallel to the road and 6 inch lateral strips that are said
to increase visibility as well as prevent damage to the center line strips. Other designs use strips that may
be laid continuously or in lengths of 12 inch to 30 inch with the width varying
from 4 to 8 inches. Depth of grooves is normally kept at half an inch, which is
considered enough to produce the rumble. Centerline rumble
strips require very little maintenance and do not contribute to degradation of
the pavement as had been the apprehension.
Centerline rumble strips can be milled on
the center of the roadway with machines
that have been specially designed for this job. Quite often painted lines are
also used in addition to the rumble strips for better visual identification.
These lines can be on both sides of the milled rumble indentation. In such
cases the width of the rumble strip is reduced. Spacing between strips can be
12 to 24 inches, though the maximum decibels which serve as warning are in
those strips that have a 12 inch distance. Length of strips in the direction of
traffic is best at 6 inches and the width across traffic 12 inches. A depth of
about a quarter of an inch in the milled surface is considered adequate to
produce the sound caused by air being forced out of the depression. Carpeting
or relaying of the surface of a road would require that these rumble strips
have to be redone. It has to be however ensured that the thickness of the new
layer is sufficient to take the new milling that is required to create the rumble
strips.