SR-SS-1 (6)
I ..55- /
LUTM:PPD: NOV 1 2 1991
w/hc.mss
COUNCIL MEETING: November 12, 1991 Santa Monica, California
TO: Mayor and city council
FROM: city Staff
SUBJECT: study Session to Discuss a Definition of Significant
Impact When Analyzing Traffic Impacts as Part of the
Environmental Review Process
INTRODUCTION
This study session report discusses criterion to determine what
constitutes a significant traffic impact under the Highway
capacity Manual (HeM) delay methodology. Four alternative
criteria for defining a significant traffic impact for purposes
of environmental review are presented, and one criteria is
identified as the preferred alternative.
BACKGROUND
On May 14, 1991 the city Council directed staff to use the HCM
delay methodology to assess traffic in the Master Environmental
Assessment (MEA) and for use in future traffic studies. The HCM
delay methodology was selected after a study session and pUblic
hearing on the various ways to prepare traffic impact reports.
The existing City significance criterion for traffic impacts is
based upon the use of the Critical Movement Analysis (CMA)
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traffic methodology. The CMA methodology focuses on the volume
of traffic that travels through an intersection in relation to
the theoretical capacity of the intersection. The HCM delay
methodology focuses on the average delay of vehicles at
intersections. Since these two methodologies are distinctly
different, it is necessary to redefine the City's definition of a
significant traffic impact for purposes of environmental review.
The civic Center Specific Plan EIR traffic study is in
preparation and will be the first traffic study to use the HCM
delay analysis and new significance criterion. The study will be
based upon the Master Environmental Assessment data for existing
and future base traffic conditions developed using the HeM delay
methodology. While a significance criterion is not required for
the MEA analysis, it is essential that the Council select a
significance criterion for use in future studies. Failure to
determine a new definition will delay the release of the Civic
Center Specific Plan Draft EIR.
ANALYSIS
DETERMINING LEVEL OF SERVICE
Under any method of traffic analysis (the previous CMA, the
current HCM, or other accepted methodology), the first step is
classifying the level of service, or LOS designation, for each
intersection. Intersections are classified as LOS A - F,
depending upon the severity of traffic conditions.
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As stated in Policy 4.3.1 of the Land Use and Circulation Element
for the City of Santa Monica:
II Safe or acceptable levels of service on city streets shall be
a criterion for evaluation of new development proposals.
Level of Service shall be "e' for collector, feeder, and
local streets and 'D' for arterials or better where
possible."
Based on this policy, intersections on collector streets
operating at a level of service A through C are determined to be
acceptable. Intersections on collector streets operating at D, E
and F are determined to be unacceptable. Under the new proposed
significance standard, an impact will be considered "significant"
if it causes an intersection on a collector street operating at
LOS A - C to operate at an unacceptable level (LOS D I E or F).
Mitigation will be required to return the intersection to an
acceptable level.
Also, intersections on arterial streets operating at a level of
service A through D are determined to be acceptable under this
policy. Intersections on arterial streets operating at E and F
are determined to be unacceptable. Under the new proposed
significance standard, an impact will be considered "significant"
if it causes an intersection on an arterial street operating at
LOS A - 0 to operate at an unacceptable level (LOS E or F) .
Mitigation will be required to return the intersection to an
acceptable level.
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Therefore, the balance of the discussion concerning significance
criteria only applies to intersections on collector streets that
will operate at a level of service D, E or F and intersections on
arterial streets that will operate at a level of service E or F.
OTHER JURISDICTION'S USE OF HCM
No guidance is found from other jurisdictions concerning a
workable definition of "significant impact II using the HCM
methodology. A number of cities throughout California currently
use the HCM delay traffic methodology for assessing operating
conditions of intersections. These cities include Rancho
Cucamonga, San Juan capistrano, Lancaster, San Jose, Berkeley,
San Francisco and Palo Alto. However, the HCM methodology is not
used by these cities for evaluating project impacts. These
jurisdictions use different methodologies or definitions of
significance for determining environmental impacts. Some cities
use a hybrid approach where HCM is used, but volume/capacity
ratios or other methods are the criteria used to determine a
significant impact.
Many cities that use HCM are located in outlying suburban areas
where, even at buildout, intersections are not expected to reach
LOS E or F conditions. This is important since, as explained
later, the seconds of delay criterion using the HCM delay
methodology cannot reliably be used once an intersection has
reached LOS F.
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Some cities in Northern California do not define a significant
impact and often do not require mitigation for projects that add
traffic to congested intersections. These cities are able to do
this since they have an extensive urban transit system available
to commuters and prefer to encourage transit use rather than
making expensive street improvements to accommodate more
vehicles.
METHODOLOGY
CAPACITY AND LEVEL OF SERVICE
The HCM delay methodology determines level of service based on
average seconds of delay. Under the City's previous methodology
(CMA) , level of service was based on capacity. Because delay is
a complex measure, its relationship to capacity is also complex.
The HCM intersection capacity analysis evaluates volume/capacity
(vIe) ratios for individual lane movements and then develops a
composite vlc ratio for the sum of critical movements or lane
groups within the intersection. The vlc ratio is the actual or
projected rate of flow on a lane or group of lanes during a peak
15 minute interval divided by the capacity of the lane or group
of lanes.
Level of service is based on the average delay per stopped
vehicle for various movements within the intersection. Delay is
a measure of driver discomfort, frustration, fuel consumption and
lost travel time. While vie affects delay, there are other
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parameters that more strongly affect it, such as the length of
green phases, signal timing, and others. Thus, for any given vie
ratio, a range of seconds of delay values may occur, and
vice-versa.
The HCM LOS criteria are stated in terms of the average delay per
stopped vehicle, and can be described as follows:
0 LOS A describes operations with very low delay, where
most vehicles do not stop at all and arrive during the
green phase. On average, vehicles wait less than or up
to 5 seconds.
0 LOS B describes where more vehicles stop than at LOS A,
causing higher levels of average delay. On average,
vehicles wait between 5.1 to 15.0 seconds.
0 LOS C describes operations where the number of vehicles
stopping is significant, although many still pass
through the intersection without stopping. On average,
vehicles wait between 15.1 to 25.0 seconds.
0 LOS 0 describes operations where the influence of
congestion becomes more noticeable with longer delays,
more vehicles stopping and the proportion of vehicles
not stopping declining. On average, vehicles wait
between 25.1 to 40.0 seconds.
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0 LOS E describes operations that are considered the limit
of acceptable delay. On average, vehicles wait between
40.1 to 60.0 seconds.
0 LOS F describes operations with delay in excess of 60
seconds which is considered unacceptable to most
drivers. On average, vehicles wait beyond 60.1 seconds.
RELIABILITY OF THE HeM DELAY METHODOLOGY
According to the Highway capacity Manual Special Report 209
(1985) which defines the HCM delay based methodology, once an
intersection reaches a vie of 1.2 or greater, the seconds of
delay produced by the model is no longer valid and cannot be used
for purposes of determining impacts at an intersection. This
situation will likely occur at most intersections in the City
with a LOS F. Thus, for determining whether a significant impact
will occur at a LOS F intersection, the seconds of delay provided
by the HeM delay methodology cannot reliably be used.
CRITICAL MOVEMENT ANALYSIS
Regardless of the methodology used, HeM or CMA, another important
factor which the City may wish to use in determining significance
is critical movements at intersections. Critical vehicle
movements are those movements that use the greatest amount of
green time at a particular signal phase. This concept is
important because vehicles can be added to non-critical movements
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without impacting the overall intersection operating conditions.
This is because the critical vehicle movements already use up
more green signal time, so the extra vehicle can move through on
the non-critical movements without needing any extra green time
to accommodate it, therefore not causing any increase in delay.
Some of the alternatives presented below take critical movements
into account when determining significance.
ALTERNATIVES
This report presents four alternative definitions to determine a
significant traffic impact. Attachmant A provides a summary
matrix of the four alternatives. These alternatives range from
the existing definition of significant impact to the most
stringent definition, which prohibits a net increase in vehicles
at LOS 0, E or F intersections on collector streets and at LOS E
or F intersections on arterial streets. Def ining a significant
impact in a strict manner does not prohibit the City from
approving development that cannot mitigate a significant impact.
Instead, a strict definition will require the decision makers to
weigh the benefits of the project against the negative impacts
and, where appropriate, adopt findings to justify the project
approval.
Since seconds of delay cannot be accurately quantified for
intersections operating at LOS F, a different significance
criterion will have to be utilized for those intersections.
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While the HCM vlc ratio is not used for determining LOS under the
HCM methodology, it still represents a tangible, quantifiable
criterion for analyzing impacts at an intersection, and is
appropriate for determining significant impacts.
ALTERNATIVE 1: NO NET INCREASE IN AVERAGE SECONDS OF DELAY
This alternative would define a significant traffic impact as any
net increase in the average seconds of delay per vehicle for
intersections operating at a future base level of service D for
collector streets and future base level of service E for arterial
streets. For all intersections operating with a future base
level of service F, this alternative would define as a
significant impact any increase of .01 or greater in the HCM vlc
ratio. Thus, this alternative uses two methods to measure the
impact due to the fact the HeM methodology does not provide
reliable seconds of delay after an intersection reaches a vIe of
1.2 or greater.
This alternative allows additional vehicles to be added to an
intersection if a project adds vehicles to non-critical movements
thereby adding vehicles but not increasing the overall vehicle
delay. This makes efficient use of the available capacity at an
intersection and represents a fair significance criteria by
emphasizing the overall operation of an intersection.
This alternative also responds to the criticism that the City'S
previous significance definition allowed projects to increase
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traffic at LOS E and F intersections by an increase in the vlc
ratio of .02 before mitigation would be required.
Finally, this alternative uses the HeM seconds of delay criterion
for determining significant impacts at LOS D and E for collector
streets and LOS E for arterial streets, and the HeM vlc ratio for
all intersections at LOS F. This provides the city with a
consistent methodology for determining both levels of service and
significant impacts.
ALTERNATIVE 2: GREATER THAN .. SECOND INCREASE IN DELAY
This alternative would define as a significant impact an increase
of greater than four seconds in average vehicle delay due to the
addition of project traffic to intersections on collector streets
with future base LOS D and to intersections on arterial streets
with future base LOS E. All intersections with LOS F will have a
significance criterion of an increase greater than .02 of the HCM
vlc ratio.
Four seconds was chosen because it is roughly equivalent to the
city's existing significance criterion under the CMA methodology.
Intersections at LOS F will continue to use the .02 allowable
increase, but with the HCM vlc ratio.
1
This alternative would allow projects to increase average vehicle
delay by 4 seconds at an LOS D intersection on collector streets
and at an LOS E intersection on arterial streets, and the vlc by
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.02 at LOS F intersections. These increases are allowed
regardless of impacts to critical movements. This alternative
uses the same significance criterion that the City previously
used, but applies it to the HeM methodology.
ALTERNATIVE 3: ANY INCREASE IN NUMBER OF VEHICLES
This alternative would define as a significant impact the
increase of one additional vehicle to any movement at an LOS D, E
or F intersection for collector streets and at an LOS E or F
intersection for arterial streets. This provides a significance
criterion that is not dependent upon seconds of delay or the HeM
v/c ratios.
This is a very strict alternative because it would not allow any
increase in vehicles even though vehicles could be added without
reSUlting in a delay to non-critical movements at an
intersection. This alternative prevents the efficient use of the
existing capacity that may exist at an intersection.
ALTERNATIVE 4: ANY INCREASE IN VEHICLES TO THE CRITICAL
KOVEKENTS
This alternative would define as a significant traffic impact any
increase in the number of vehicles to a critical movement at an
LOS D, E or F intersection for collector streets and at an LOS E
or F intersection for arterial streets.
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Like alternative number 3, this alternative relies solely upon
the number of vehicles added and is not dependent upon seconds of
delay or HCM v/c ratios. The theory behind using the number of
vehicles added to the critical movements is that those are the
movements that are significantly impacting an intersection.
Additional vehicles can be added to movements at an intersection
where they will not result in any additional delay.
NOTIFICATION
A notification flyer was mailed to a list of over 500 interested
parties notifying them of the November 12, 1991 study session.
PREFERRED ALTERNATIVE
Staff will return to eouncil with recommendations to be
considered at the November 19, 1991 meeting. At that time,
alternative one will be presented as the criterion to determine a
significant traffic impact. This alternative defines a
significant impact as any net increase in average seconds of
delay per vehicle for intersections operating at a future base
LOS D for collector streets and at a future base LOS E for
arterial streets. For all intersections operating at a future
base LOS F, this alternative defines a significant impact as any
increase of .01 or greater in the HCM vlc ratio.
This alternative is preferred because it provides for essentially
a no net increase in the HeM seconds of delay and vlc ratio for
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intersections operating at an unacceptable level; future base LOS
D, E or F for collector streets and future base LOS E or F for
arterial streets. Yet this alternative allows for efficient use
of the available capacity at an intersection by allowing vehicles
to be added to the non-critical movements as long as there are no
significant impacts associated with those additional vehicles.
Also, by using the HeM seconds of delay and vjc ratios, this
alternative provides the eity with a consistent methodology for
determining both levels of service and significant impacts.
staff also proposes that eity council policy define as
"significant" an impact which causes an intersection on a
collector street operating at LOS A - e to operate at an
unacceptable level (LOS D, E or F) and an impact which causes an
intersection on an arterial street operating at LOS A - D to
operate at an unacceptable level (LOS E or F). Mitigation will
be required to return all impacted intersections to an acceptable
level.
Where mitigation is not feasible, unavoidable significant adverse
impacts will result. The Planning eommission, or eity eouncil on
appeal, would be required to adopt a statement of overriding
considerations finding that the benefits of a project outweigh
the adverse environmental impacts if a project is to be approved.
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Prepared By: Paul Berlant, Director of LUTM
Suzanne Frick, principal Planner
Liz Casey, Associate Planner
Paul casey, Transportation Planner
Land Use and Transportation Management Department
Program and Policy Development Division
Attachment A: Alternatives for Selection of a Significance
eriteria (Matrix)
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