HomeMy WebLinkAbout06_28-29_2017 PC Correspondence - Avila Ranch (Foley)Narrow Residential Streets:Do They Really Slow DownSpeeds?
James M.Daisa,P.E.and John B.Peers,P.E.
Introduction
Transportation planners and traffic engineers are often asked to consider designing
narrow residential streets or narrowing existing wide residential streets as a measure
to reduce speeds.Only one of a litany of traffic calming measures,narrowing streets is
almost taken for granted to be an effective method of slowing traffic.In addition to
literally reducing the curb-to-curb width through design or retrofit,there are several
common ways to physically narrow sections of streets including the installation of
chicanes,necked curb returns,and tree planters in parking lanes.Some communities
Vehicle Stopping Distance
Speed
I /
Perception
Reaction I
~.–-1_I /148’Fatal I
I 4 .,-.,/\!%rinl IS I---,---
20 mph.~
107’196’313’
Total Stopping Distance
Figure 1.Pedestrian and Bicyclists Accident
Severity with Vehicles at Various Speeds
speeds in the low to moderate injury range (20
indicate success in “perceptive”
narrowing of wide streets through
painting edge lines or adding
bicycle lanes.
There are clear and obvious
benefits of slowing traffic on
residential streets,primarily the
improvement of pedestrian and
bicyclist safety.Slower traffic
reduces the severity of accidents,
reduces noise,and generally
improves the livability of residential
streets.Figure 1 shows the
relationship between vehicle speed
and stopping distance,and
pedestrian and bicycle accident
severity.Attempting to achieve
mph or less)along the entire length of
a residential street is certainly desirable from a pedestrian and bicyclist safety
perspective.
When considering narrowing residential streets as a traffic calming measure,it is
reasonable to ask the following questions:
l Is street narrowing alone an effective calming measure?
l How narrow must the street be to dramatically reduce speeds?
l What other factors affect residential street speed,and what is their relationship?
Data Collection
This article presents research into the effect of width on residential street speeds in the
San Francisco Bay Area.Speed data was collected on nearly 50 streets with curb-to-
curb widths varying from 25 to 50 feet.In addition to speed and width,parking density
was surveyed on a number of the streets.Selected street segments met specific
criteria to ensure speed data was consistent and comparable.The criteria were:
.Meets California Vehicle Code definition of a residential district.
c Relatively straight and flat or with low grades.
“Provides for through traffic.
l No existing traffic calming devices such as speed humps,etc.
l Not within a school speed zone.
l Average daily traffic volumes less than 5,000.
.Parking permitted on both sides of the street.
Summary of Conclusions
The conclusions of the research are based on analysis of the collected data and
observation of speed behavior on streets with various traffic volumes,headways,and
parking densities.The key findings of the research are:
1 )Wider residential streets experience higher speeds for both the average and 85th
percentile speeds.
2)On-street parking density significantly affects speeds.
3)Traffic volume and vehicle headways affect speeds.
4)Significant reductions in “effective”street width are required to dramatically reduce
speeds.
Speeds Versus Street Width
28-
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27-
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o
0 q
1%
a 26 .
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m
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<25 .
q
R2=0.902
24-
25-29 30-34 35-39 40-44 45+
Street Width (ft)
Figure 2.Average Speed vs.Street Width
Group
Figure 2 presents average speed by
street width groups.The points on
the graph reflect the average of the
speeds within each of the width
groups.The data shows a slight
increase in average speed as street
width increases,about 4 mph
between the narrowest and widest
groups.While there is good
correlation between average speed
and width group,the individual
speeds within the width groups form
a broad range with little correlation
to street width.
Figure 3 presents 85th percentile
speed by street width group.Similar
to average speeds,the 85th
percentile speeds increase slightly
with street width,about 3 mph between the narrowest and widest groups.The
average 85th percentile speed levels off between 31 and 32 mph as street width
excee~s 35 feet.While the-data indicates that,
z
g
u
al
a!
1%
q R2=0.784
25-29 30-34 35-39 40-44 45+
Street Width (ft)
Figure 3.85th Percentile Speed vs.Street
Width Group
40
35
30
25
20
R2=0.601
q
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q
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n
q q q
q
q
q
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q
q q
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o 10 20 30 40 50 60 70 80
Opposing Volume (vph)
Figure 4.Average Speed vs.Opposing
Traffic Volume
in general,speed decreases slightly as
street width decreases,there isn’t a
strong correlation between speed and
street width alone.Other factors must
affect speed as well.
Speed Versus Opposing Traffic
Volume
Figure 4 present average speed
versus opposing hourly volume.The
data points reflect streets of varying
widths with the lowest speeds on the
narrowest streets with the highest
opposing volume.
There is a relatively strong correlation
between average speed and
opposing volume,particularly on
narrow streets where drivers either
must pull over and stop to let other
vehicles pass or where the perception
of street width is too narrow to judge
accurately.In either case,on-street
parking density plays an important
role as it defines the effective width of
the street.Related to the opposing
volume is the headway between
vehicles.Short headways mean that
drivers encounter other vehicles more
frequently and are thus required to
slow down more often.Even in the
same direction of travel,short
headways influence the speed at
which drivers travel.Observation has
shown the situation which promotes
the highest speeds on residential
streets is a wide street with low
parking density,low traffic volumes,
and long headways between
vehicles.This situation,which often
occurs at night,minimizes the number of vehicles (parked or moving)which can
influence a drivers perception of their own speed based on the proximity of other
vehicles.
Speed Versus Parking DensityOneofthemostinfluentialfactorsin residential street speed is the density of on-street
parking,especially on narrow streets.On-street parking on both sides of the street
defines the “effective”width of the street.On narrow streets with a relatively high
density of parking,the effective width can be as narrow as a single lane forcing a
driver to pull over and stop when an opposing vehicle is encountered.Figure 5
presents average speed versus on-street parking density.The points on the graph
o 100 200 300 400 500
Parking Density (veh/mile)
Figure 5.Average Speed vs.On-Street Parking
Density
Wide streets with low parking density (Figure 6)
have a wide effective width and virtually no
calming effect.Without the influence of other
moving or parked vehicles,this width of street
promotes speed.Wide streets with high parking
densities (Figure 7)provide a narrower effective
width resulting in a low calming effect.Narrow
streets with low parking density (Figure 8)have an
effective width similar to wide streets with high
parking density,but produce a moderate calming
because off-set parked vehicles create a chicane
effect.
Finally,narrow streets with high parking density
(Figure 9)have the highest calming effect because
it reduces the width of the street to a single lane.
This forces drivers to pull over and stop to allow
opposing traffic to pass.While this situation
represent very low to moderately
high parking densities on streets
of varying widths,both during the
day and at night.As stated above,
high parking densities on narrow
streets which reduce the effective
width can dramatically slow
speeds.
Effective Street Width and
Parking Density
The following examples illustrate
the calming effect of the “effective”
street width as a function of curb-
to-curb width and parking density.
Traffic volumes and headways
also contribute to the calming
effect.
Figure
Street
lWide Street
lLOW Parking Density
.No Calming Effect
6.Effective Width of Wide
with Low Parking Density
Figure
Street
lWide Street
lHigh Parking Density
lLow Calming Effect
7.Effective Width of Wide
with High Parking Density
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lNarrow Street
lHigh Parking Density
lHigh Calming Effect
Figure 9.Effective Width of Narrow
Street with High Parking Density
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lNarrow Straet
lLOW Parking Density
lModarata Calming Effect
(Chicane Effect)
Figure 8.Effective Width of Narrow
Street with Low Parking Density
creates an effective calming measure,it
cannot be sustained over a long
segment of street.Breaks in the parking
density,or numerous driveway curb-
cuts,are required to provide space for
drivers to pull over to allow vehicles to
pass.
Hypothesis of a Calmed Street
Analysis of the data presented above
and observations of residential street
speed characteristics leads to the
formulation of a hypothesis for
narrowing streets as a traffic calming
measure.Figure 10 illustrates the
relationship between street width,traffic
volumes,and calmed streets.Relatively
high parking density is assumed as a component of this relationship.Within a range of
street widths and traffic volumes (or headway between vehicles)a street is calmed,as
depicted by the shaded portion of the graph.The calmed street area is roughly
bounded by widths less than 36 feet wide and headways less than 30 seconds in the
peak hour (equivalent to about 1,500 to 1,600 vehicles per day).As width and
headways increase drivers are not inhibited by width and speeds,therefore,become
independent of width.
Since it is impractical and undesirable to decrease headways (by increasing volume),
street width becomes the variable to work with.For streets to operate well within the
calmed area the lower limit of volume is roughly 160 vehicles per hour (1 ,600 vehicles
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240 160 120
(15.0)(22.5)(30.0)
Vehicles Per Hour
(Vehicle Headway in Seconds)
Figure 10.Calmed Street Hypothesis
per day)or headways between vehicles of
22.5 seconds,and the upper limit of the
street width is about 28 to 30 feet (14 to 16
feet effective width).Further study is
required to develop a mathematical model of
speed as function of street width and
volume which can be used by planners and
engineers to design new streets and retrofit
existing streets to the desired effect.
Methods to Reduce the Effective
Width
The effective width of a residential street can
be narrowed using several common
techniques.Existing wide streets may be
narrowed with tree planters in parking lanes,
raised medians,curb bulb-outs at intersections and mid-block,chicanes,or slow
points.These devices should be spaced appropriately to maintain low speeds along
the entire length of a street.To effectively lower speeds in the design of new streets the
curb-to-curb width should be 28 to 30 feet with parking allowed on both sides.Off-
street parking requirements can be reduced to promote higher on-street parking
densities.
Trade-Offs of Narrowing Residential Streets
The desire to reduce speeds on residential streets through narrowing effective width
must be balanced with several disadvantages.Narrower streets are often
discouraged for the accommodation of emergency vehicles,garbage trucks,and other
large vehicles.Driver visibility of pedestrians may be reduced,especially with high
parking densities where children may dash out from between parked vehicles.
However,lower speeds reduce the required stopping distance.Sideswipe accidents
with parked vehicles may increase.However,older neighborhoods with narrow
streets have existed for many decades,generally without problems.Despite the
disadvantages,many communities strive to reduce speeds,improve pedestrian and
bicycle safety,and improve the livability of residential streets through traffic calming
measures.Narrowing residential streets can be one of many effective measures to
consider.
Author Information
James M.Daisa,P.E.is an Associate with Fehr &Peers Associates,Inc.,Lafayette,CA
94549,(51 O)284-3200,Member ITE
John B.Peers,P.E.is a Principal and co-founder of Fehr &Peers Associates,Inc.,
Lafayette,CA 94549,(51 O)284-3200,Member ITE