|
Space signs at a distance from each other. When they
are clustered, confusion results. Keep signs at least 150 feet apart when
possible.
Never locate a sign with its post placed within a
shoulder lane where a driver may be forced to pull over.
Avoid placing signs on curves or lane drops where
they will be hard to see.
When you have to use a post-based sign, set it into
the ground. Don’t encase its base in concrete since this can damage a
motorist or cyclist who hits it.
Visibility
In addition to basic positioning rules, sign
visibility and the resulting safety also depend on positioning for good
retroreflectivity — day or night.
In a major study conducted by the Federal Highway
Administration at the Texas Transportation Institute, nighttime field
testing provided agencies with guidance for visibility needs.
Typical sign positioning was used, with the bottom
of overhead signs set at 18 feet above the street. On posted signs, the
bottom of the sign was 9.5 feet above the street surface.
Overhead signs were centered over the travel lane.
Posted signs were placed 6 feet right of the right-hand edge of the travel
lane.
Drivers of cars, SUVs, and trucks all have differing
levels at which they can most easily see a street sign. Unfortunately, they
are not the same. A driver’s eyelevel in a car, for instance, will be about
3.5 to 3.75 feet. In an SUV or pickup, he or she will have eyeline height of
4.6 to 4.9 feet. In a heavy truck, the driver’s line of sign is about 8
feet.
Study results show that use of Type III (or higher)
retroreflective overhead sign material provides adequate luminance in the
travel lane for cars when the sign is 25 feet above the street and the
vehicle 640 feet or less from the sign. Vehicles with taller lines of sight
and headlight heights need more reflective sign material.
In another TTI study, cities responding to a survey
about signs made several important points about visibility:
-
Green EC film on high-intensity sheeting, 3M’s
Visual Impact Performance sheeting, high-intensity sheeting, and
diamond-grade VIP sheeting were commonly suggested as materials to use.
-
High-intensity on red or yellow background and
engineering grade for white/green/blue ground-mount signs were
recommended.
-
Sign heights to the bottom of the overhead sign
ranged from 16 feet to 20 feet, 9 inches from the street surface.
-
Ground-mounted sign heights ranged from 7 to 10
feet among the towns and cities responding.
-
Most towns and cities follow state MUTCD
specifications, which are most often patterned on federal MUTCD specs.
-
Overhead signs are usually mounted on signal
mast-arm poles.
-
Most cities and towns use white letters on a
green background for street signs.
-
Black letters on a yellow background are
generally used for advance warning signs.
-
Letter height generally average 6 inches for
lower-case letters and 8 inches for capital letters.
-
FHWA Series B or C alphabets are most often
used.
 |
 |
 |
| This sign (top) is confusing and unsafe
because it includes two versions of Voight Drive. The smaller
letters, explaining that the street is a loop would be
unreadable to the motorist. The second sign (next to the top),
is clear and legible and provides safe communication if
positioned correctly. The bottom sign, with icy branches over
it, is not safe because the branches obscure its message.
|
The FHWA has taken other steps to improve sign
visibility. Mobile vans can be used to measure the retroreflective qualities
of pavement markings. Private industry makes and sells these units and
provides contractual support for their operation and maintenance. Similar
vans for measuring the retroreflective qualities of signs are now being
tested by the FHWA.
There has also been preliminary research on the use
of ultraviolet headlights. These allow drivers to use a low-beam level, yet
see fluorescent traffic control devices as if they had their high beams on.
Phosphorescent materials and LED lights used to
augment pavement markings are other areas being studied.
Technology use
Sign Inventory Management Systems help towns,
counties, and cities identify unsafe traffic signs so that they can repair
or replace them.
In New Hampshire, towns use a SIMS that includes
sign condition assessment, priority analysis of treats to motorist safety,
repair and maintenance scheduling, and work documentation of projects
completed.
Utah has an asset management package that lets towns
and cities use a GIS-based system to evaluate signs, predict safety needs,
and so on.
Des Moines, Iowa is testing use of a typeface called
Clear, which is reportedly easier to read at a distance.
The FHWA’s MUTCD update includes recommendations
for the use of fluorescent pink signs to alert drivers to traffic
information, to increase sign letter sizes, and to use animated-eye signs to
remind pedestrians to look both ways before crossing.
In Pittsburgh, some street intersection signals use
recorded walk/don’t walk instructions to supplement visual signals.
Unfortunately, with even moderately heavy traffic, it is not possible to
hear the instructions, making the value for blind or visually handicapped
pedestrians minimal.
Reprinted from Better Roads Magazine
September 2005 |
