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Specifications and Layout Considerations |
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Tolerances | "Continuous"
vs. "Skip Pattern" Layout Designs
Maintenance & Protection of Traffic
Specifications | Clean Up Specifications
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Click here for a printable
version of the illustration above.
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| Tolerances |
It is critical that specifications call for a "minimum
1/2-inch depth to a maximum of 5/8-inch depth," rather
than calling for a 1/2-inch deep cut with a 1/8-inch tolerance.
1/8-inch equates to 25% of the milled depth. If a highway
does not demand a minimum 1/2-inch depth, contractors will
mill shallow 3/8-inch cuts, which will not alert truck drivers
and other vehicles drifting off the road.
Likewise, specifications should call for minimum dimensions
of 7-inches in the direction of travel. Specifications must
allow for some tolerance, however, highway authorities should
not accept any cuts less than 7-inches in the direction of
travel. Large radius tires, such as truck tires, will simply
straddle narrower cuts and they will not generate sufficient
noise and vibration.
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| Offset from the Edge Line |
Highway authorities have varied the offset of the rumble strips
from the edge line. Some states have placed the rumble strips
directly on the edge line (and later painted through the cuts),
while others have offset the rumble strips anywhere from 4
to 18-inches.
Designers should consider safety, maintenance, and shoulder
use concerns in determining the offset of the rumble strips
from the edge line. The closer the rumble strips are to the
edge line, the sooner they will alert a drifting driver, allowing
him more time to recover.
From a construction standpoint, the only consideration regarding
offset is in those instances where the rumble strips are placed
on a narrow shoulder adjacent to a guardrail or a concrete
barrier. In these instances, designers need to make sure they
have allowed enough room for a rumble strip mill to work between
the location of the cut and any vertical barrier (nearly all
mills can fit in areas of 5-feet or more).
The
air turbulence from the passing traffic keeps the rumble strips
blown clear from a build up of debris, sand, or water. This
air turbulence decreases as the rumble strips are moved further
off the edge line, thus allowing the rumble strips to fill
up and become ineffective.
Also, rumble strips placed close to or on the edge line maximizes
the available usable shoulder.
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"Continuous" vs. "Skip Pattern" Layout Designs |
Most states have adopted a continuous layout design for shoulder
rumble strips. This has provided states with a consistent
and effective layout design, which specifies that the rumble
strips be placed on continuous 1-foot centers along the edge
line of the shoulder.
An alternative design, however, has become increasingly popular
in recent years. The "skip" pattern uses the same
7" x 16" x 1/2" cuts placed on 1-foot centers,
but with an occasional gap. Below is an example of a “skip
pattern” or “gap” design”:
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Most state highway authorities that have tested both designs
have gone on to use the skip pattern rumble strip. State
engineers have cited three reasons for their preference
for the skip pattern:
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The skip produces a more alarming sounding that may do
a better job of alerting inattentive drivers.
- The skip pattern requires less milling and so is slightly
less expensive.
- For cyclists who prefer not to traverse the rumble strips
when crossing the road, the gap provides them an easy
area to traverse.
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Maintenance & Protection of Traffic Specifications |
- The travel lane adjacent to the shoulder must be closed
off during rumble strip construction.
- Most rumble strip operations move from 1-2 miles per hour,
often cutting over 10 miles per day on asphalt.
- Moving lane closures with at least two crash trucks has
proven itself the safest and most cost effective method
of traffic control during rumble strip construction. For
safety reasons, most state highway authorities will not
allow static lane closures with cones or barrels during
rumble strip construction.
The file below (PDF format) shows a M&PT plan for rumble strip
construction:
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| Clean Up Specifications |
While the vast majority of highways (over 90%) allow contractors
to waste the milling debris over the shoulder's edge, some
states require that contractors pick up the debris and haul
it off the project to an appropriate dumpsite.
Due to the considerable cost of removing the debris from the
job site, it is critical that the specifications make clear
how the clean up of the debris is to be handled. Specifications
which leave it up to the "engineer's discretion"
invariably result in delayed bid dates and addendums until
the engineer makes the requirements clear.
If the state does not wish to have contractors sweep the debris
over the shoulder's edge, they should state this clearly,
such as:
"The contractor must pick up and haul the millings off the
roadway to a suitable dump site. The contractor will not
be permitted to waste the millings over the shoulder's
edge."
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