A Speed Reduction Strategy in Local Street Systems - Institute of ...

Humps - A Speed Reduction Strategy in Local Street Systems
Sue DattaaandTapanK. Datta,Ph.D., P.E.’
Residential area speed control has been a critical issue for local
Trat%c Engineers for several decades. Engineers have
attempted to utilire acceptable freatrnents such as road clsum,
diverters, speed humps and roundabouts, to some
unacceptable treatments such as midblock and unwarranted
stop signs. These efforts have met with varying levels of
success in the past.
Itis also importanto note that the speeding on local street
systems and cut-through tile in our neighborhoods are the
two most important concerns in many suburban CQmmunides
m the USA. Traftlc Engineers ofteu spend countless hours
either studying and/or answering citizens’ complaints aud
concerns on such issues. Traffic offkials in a local comrmmity
have very few options in terms of implementing countermeasures+for
such a traflic problem in our neighborhoods.
The elected officials in many suburban cmunuuities often
satisfy pUbliC demand to mitigate speeding and cuMhrough
trafllc problems by approving unwarranted stop signs and fourway
stop signs. Past studies have indicated that such measures
do not work unless there is some sort of sight restriction at
local street intersections. Engineers recently raised issues with
our current stop sign guidelines and theii effectiveness.
Studies are required to prove or disprove the effectiveness of
existing tw~way stop signs and four-way stop signs, warrants
and guideJi.nes, as per the Manual of Uniform Traffic Control
Devices ~). N tilC Officid$ are under Constant
pressure fforn the elected officials’ popular and indidmimmt
use of stop Signs.
Local area speed control and @-through traftIc are often
related issues and require careful study. If a residential area
road provides an alternative to congested arterial streets, there
will be both speeding and cut-through trat%c in our neighborhood
Street SySteIU. MSny cOUntlieSin tiOpe SUd SkO iU
Australia traffic officials have aggressively pursued various
physical freatrnents in their local street systems. They include:
* Roundabouts,
● Speed humps,
●
●
●
●
●
Road ChSUK!S,
Dips,
Diverters,
chokers,
and other physical mitigation measures.
These treatments are referred to as traflic calming devices.
Various cities in the USA have been experimenting with many
of these measures to slow down and reduce traffic in our
residential areas.
Residential collector roads, which are often long, particularly
in older subdivisions with grid type or near grid type road
layouts, can provide an alternative means of through travel. If
we measure the speeds, we fd the 85th percentile to range
ffom 30 to 45 mph. However, does this mean we ought to
change speed limits to higher ones utilizing the 85th percentile
speed? Probably not considering the nature and dominant
fimction of the collector and local roads. A reasonable and
intelligent option would be to keep such areas at 25 mph,
while developing and installing physical measures which Wti
face motorists to slow down. This brings up another issue
which is as controvemial as residential area traffic control.
This is the issue of setting speed limits based on 85th percentile
speedonly. Somestatesutilize geometic and functional
factow in additionto, the85th pementilespeedasthedeterminant
of speed limits. Gthers have only followed the 85th
percentile speed for setting speed lids.
The objective of tids study was to evaiuate if the installation
of a physical measure such as a speed hump signitkantly
reduces operating speeds and tile volumes.
A speed hump is a raised area in the roadway surface extending
the entire travelway with the crest height behveen three to
four inches, and a length of approximately twelve feet, The
higher the crest height and shortti the length of the hump, the
worse is the driver and passenger discomfort. The underlying
‘ Transportation Planner, Goodell-Grivas, Inc.
b Professor, Department of CiviI and Environmental
Engineering, Wayne State University, ITE Member
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principle of the hump design is to force motorists to reduce
speed to a level of about 15 to 20 mph. ‘lEus, if a vehicle
drives over a speed hump at the speed limit of 25 mph for a
residential area, the driver will feel discomfort, yet will not
lose control of the vehicle.
Some speed humps may have a parabolic shape or sometimes
have a flat surface on top. The later type of humps are
COIWOdY used in Austmlia and OCG&OIEd& in the USA.
EXPERIENCE
OF SPEED HUMPS
lleuseofspeed hurnpsas a~ccahningdevice inconsistently
gaining popularity in the USA. Although, Europe and
Australia continue to lead the way in experimenting with and
installing speed humps as a traffic calming device.
The tests conducted in England involved various sizes of speed
humps. After much study, they decided to use 12-feet wide
and 4-inch high humps. They also tested various spacings that
ranged from 160 feet to 510 feet between the humps.
Several cities in the North America repmted use of humps as
trsfilc cahning device. The City of Thousand Oaks, California,
has tested the use of humps extensively. The results of
their before and atler study is as follows
II Site I 85th Percentile Speed II
1. Langtry Drive
2. Clarice Avenue
3. Campbell Drive
4. 130nanza Road
5. AvaIon Road
6. Pyramid Drive
AkhouglL there were other sites where speed humps have been
installe~ these are the locations where there is both before and
afier data available. The after speed data has been verifti by
the authors, by cdecting sample speed data during off-peak
periods using a radar gun in the f~ld.
Table 1 shows the before and after speed and volume data at
the six study sites in Las Vegas, Nevada.
Table 1. Before and after speed and volume data.
Before
Site = ‘
ADT 85# ADT 85#
speed
speed
Langtry Dr. 25 238 29 191 23
CIarice Ave. 25 3,047 38.12 3,316 26
Campbell Dr. 25 4521 - 2,233 27
Bonanza Rd. 25 3,006 34 3,150 27
Avalon Ave. 25 3,455 37.50 2,040 21.86
Pyramid Dr. 25 2,753 29.02 - 23.1
A study in the province of British Columbia, Canat@ reported
benefits accrued from accident reduction in tangible terms in
theuseof various traffic calming devices. Inallcases, the
dollar benefits due to the instaktion of various traffic cahning
devices far outweighed the cost of installation just from the
reduction of accidents.
Such eqeriences have led the authorsof this paper to conduct
a study to test the statisticalsignificanceof speed and traffic
volumereductionin the residentialareasdue to the imtallation
of speed humps. Severalspeed hump sites were selected in
the City of Las Vegas,Nevadato perform this study.
Thebefore speed data for Campbell Drive and after vohnne
data for Pyramid Drive were not available.
The percent changes in speed and trafilc volumes at the six
study sites are as follows
Ii site I Percent Reduction m n
II i 85th % Speed I Am II
Langay Dr. 1 20.6% I 19.7%
STUDYSITES, SPEED AND VOLUME DATA
Rec%ntly the C@ of Las Vegas has been instaMg speed
humps in their residential areas quite aggressively. The
following sites in the City of Las Vegas were selected as a part
of this study
m ,
Avalon Rd. I 41.7% I 40.9% 11
I
20.4% I II
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Figures 1,2 and 3 show the locations of the study sites in the
context of the adjacent roadways.
BARNARD
.(J b-l
SERAFIN
B
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3
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3 VALMOR
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Figure 1.
I
.
TORREY PINES DR.
Figute 3.
I
The 85th percentile speedchangesobservedin the six study
sites in Las Vegas we compared with observed speed changes
in other communities and those reported in the past. The mean
pment change in the 85th percentile speed in the six sites in
Las Vegas is 27.02 percent. This compares favorably with
other experiences. 30 percent reductions on an average, was
observed in tests performed in England
A befote and after speed study performed for a speed hump
location in Kansas City, Missouri showed a reduction of the
85th percentile speed fmm 33 mph to 25 mph, thus a reduction
of 24.2 percent was observed. The average of the two sites m
the City of Thousand Oaks, California, reported an overall
reduction of the 85th percentile speed of21 percent. l%erefore,
the speed reduction observed m the Las Vegas sites is, as
expected in comparison with the other studies performed in the
USA and abroad
Figure 2.
The reduction in Mlc volumes due to the installation of
speed humps can only happen if a roadway is carrying cutthmugh
traffic. Cut-through traffi is defmd as the traftic
whose origin and destination are both outside the neighborhood
which is under consideration. A study of the Average Daily
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Traffic (ADT) at the five out of six sites in Las Vegas
revealed that three out of five sites reported approximately a
20 to 50 percent reduction in ADT. Two sites recxxded
approximately 5 to 9 percent increase in traffic volumes. The
Kansas City site recorded a slight increase in tmffic volumes.
A study of the test sites in Las Vegas indicated that the trafllc
volume change may be due to other factors besides the
installation of speed humps.
HUMP INSTALLATIONS
The use of humps in the USA, while not as popular as in
England, is still gaining popularity rapidly. In genemi, most
communities in the USA seeking to install humps on their
problem-plagued residential streets, are coming from the
citizens and elected offkials. Engineering and enforcement
officials oftentimes are opposed to smh barriers. However,
under political pressure many cities are installing such humps
and evaluating the results carefully.
The following speed hump characteristics have a direct effect
on whether or not the overall speed in a residential street is
going to be reduced
1. Size of the hump crest height.
2. Length of the hump.
3. Len@ of the roadway.
4. Spacing of humps.
Various tests around the world have more or less standardized
the crest height and the length of speed humps. However, the
spacing between humps has still not been tested objectively
and remains to be an approximation. The basic premise for
de@mining the optimal spacing between speed humps should
include the-following: - - - -
Therefore, the installation of warning sign and the next hump
should consider the approach speed of the vehicles to the
section of roadway where there are a group of humps to be
placed. The rate of deceleration by the road users and rate of
acceleration after encountering a hump are also determinant
factors for optimal spacing of humps.
The advance warning signs are an essential part of the hump
installations. Pavement markings are also quite helpful and
aid the motorists in wqn.ition of the situation.
SIGNIFICANCETESTTNG
The before and after data from various cities as reported in
papers, internal studies and as observed by the authors, all
point to substantial reduction in speeds at the midpoint between
humps as well as overall speed. The following before and
after speed data were used for a signifkance testing using the
“t” test:
City Roadway 85th Percentile
s~
Before
(mph)
(:p;)
Las Vegas, Langtry Dr. 29 23
Nevada
Clarice Ave. 38.12 26
Campbell Dr. Not 27
Available
Bonanza Rd. 34 27
Avalon Rd. 37.5 21.86
1.
2.
3.
4.
Drivers slow down at humps, however, they start
accelerating as soon as they cross a hump to makeup
for their lost time.
The sign for the next hump should be placed at a
point so that it is visible by the driver just about the
time it reaches 25 mph speed or less.
The placement of the sign will make the driver return
to deceleration mode expecting the next hump to
arrive soon.
The placement of the next hump should consider the
distauce a typical car will lraverse at a normal rate of
deceleration to bring the vehicle to a much lower
speed.
-d Dr. 29.02 23.1
I’housand Kelly Rd. 43 30
Oalrs,
California Silas Ave. 38 34
Kansas City, 7oth St. 33 25
Missouri
Farmington Lancaster Dr. 35.5 26
Hills,
Michigau Kendallwood Dr. 35.7 30.3
Hearthstone Dr. 37.4 28.7
The sample means and standard deviations of the before and
after 85th percentile speeds at the above noted locations are
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~ before = 35.48 mph
u before = 1.3
~after =24.33 mph
a after = 1.05
Nbefore= 11
Nafter =12
d.f =11+12.2=21
t Calc. = 17.876
t critical = 2.831 at 95% level of cdldence
Therefore, the conclusion is that the 85th perctmtile speed is
reduced signifkantly at the 95 percent level of confidence and
that the speed reduction is attributable to the installation of
speed humps. This reduction was observed irrespective of the
location with the USA.
CONCLUSIONS
AND RECOMMENDATIONS
The study of speed humps at various locations within the USA
revealed the following
1. The speed at midpoints between the humps and the
overall speed reduction was due to the installation of
speed humps.
2. Traftlc volumes also reduced in some locations,
however in others either it remained the same or
increased slightly. The opportunity to divert to an
alternate route is probably the key factor tiecdng
traffic volume changes.
3. While most agree that a 12-foot by 3- to 4-inch
speed hump is reasonable, various agencies continue
to modify them, to make them less drastic by using
only a 3-inch crest and increasing the length of hump
to 14 feet to 16 feet
8. Speed humps as a speed reduction/trafllc calming
device seems to be working in many different
locations and situations.
ACKNOWLEDGEMENT
The authors would like to acknowledge
cooperation of the following agencies
the assistance and
City of Las Vegas - Trafllc Department, Las Vegas,
Nevada
Metro Police Department, Las Vegas, Nevada
City of Kansas City, Missouri
City of Farmington Hills, Michigan
The agencim have provided their data, which helped the
authors in the preparation of this paper.
BIBLIOGRAPHY
1. Las Vegas, Neva&, A Neighborhood Trafic Management
Program for the City of f.us Vegas, May 1995.
2. Institute of Transportation Engineers (ITE) - ITE Technical
Council Speed Humps Task Force, Guidelines for the
Design and Application of Speed Humps, August 1995.
3 Zein, Sany R., Geddes, Erica and Hemsing, Suzanne,
Safety Benefia of Tram Calming, Paper presented at
1997 TRB Conference, Washington, D.C., January 12-
16, 1997.
4. The spacing between humps remains a vtible
without much uniformity among various users.
5. Further study is necesmy to develop a relationship
between approach speed, desirable midpcimt speed,
hump cross-section and length.
6. Use of grade and/or alignment may not be necessary
since approach speed observation will include the
effect of such geometric characteristics.
7. In many instances, the Fire Department and the
Police Department do not advocate the installation of
speed humps, citing the effect that they slow down
their fire and police cars. However, there is no
evidence that such a slowdown has caused loss of
life or property.
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