SR-9-A (88)
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LUTM:PPD:
w{hcmsigms
COUNCIL MEETING: November 19, 1991
Nm, 'r. -,,'1
Ut .: _~_ oJ;/;
Santa Monica, California
TO:
Mayor and City council
FROM:
City staff
SUBJECT: Recommendation to Select a Definition of Significant
Impact When Analyzing Traffic Impacts as Part of the
Environmental Review Process
INTRODUCTION
This report recommends that the city Council select a criterion
to determine what constitutes a significant traffic impact under
the Highway Capacity Manual (HeM) delay methodology. This report
presents four alternative criteria for defining a significant
traffic impact for purposes of environmental review, and
recommends one 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 HeM
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)
9~A
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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 HCM 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:
" Safe or acceptable levels of service on City streets shall be
a criterion for evaluation of new development proposals.
Level of Service shall be 'c' 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, E or F).
Hi tigation 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 - D 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 HeM 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 HeM 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
(v/c) ratios for individual lane movements and then develops a
composite v/c ratio for the sum of critical movements or lane
groups within the intersection. The vIe 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,
green phases, signal timing, and others.
ratio, a range of seconds of delay
vice-versa.
such as the length of
Thus, for any given v/c
values may occur, and
The HCM LOS criteria are stated in terms of the average delay per
stopped vehicle, and can be described as follows:
o 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.
o 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.
o 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.
o LOS D 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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o LOS E describes operations that are considered the limit
of acceptable delay. On average, vehicles wait between
40.1 to 60.0 seconds.
o 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 HCM DELAY METHODOLOGY
According to the Highway Capacity Manual Special Report 209
(1985) which def ines the HCM delay based methodology, once an
intersection reaches a v/c 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 HCM 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 D, E or F intersections on collector streets and at LOS E
or F intersections on arterial streets. Defining a significant
impact ln 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
intersections operating at LOS F,
criterion will have to be utilized
accurately quantified for
a different significance
for those intersections.
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While the HCM vjc 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 v/c
ratio. Thus, this alternative uses two methods to measure the
impact due to the fact the HCM methodology does not provide
reliable seconds of delay after an intersection reaches a v/c 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 v/c
ratio of .02 before mitigation would be required.
Finally, this alternative uses the HCM seconds of delay criterion
for determining significant impacts at LOS D and E for collector
streets and LOS E for arterial streets, and the HCM vjc 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 4 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
vjc 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 v/c ratio.
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 v/c 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 HCM 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 HCM
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-cri tical 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
MOVEMENTS
This alternative would define as a significant traffic impact any
increase in the number of vehicles to a critical movement at an
LOS 0, 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.
BUDGET/FINANCIAL IMPACT
The recommendation presented in this report does not have a
budget or fiscal impact.
NOTIFICATION
A notification flyer was mailed to a list of over 500 interested
parties notifying them of the November 19, 1991 eity Council
meeting on the selection of a significant traffic impact for
purposes of environmental review.
RECOMMENDATION
It is recommended that the City Council adopt alternative one 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
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future base LOS F, this alternative defines a significant impact
as any increase of .01 or greater in the HeM vlc ratio.
This al ternati ve is recommended because it provides for
essentially a no net increase in the HCM seconds of delay and v/c
ratio for intersections operating at an unacceptable level;
future base LOS 0, 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 HCM seconds of delay and
v/c ratios, this alternative provides the City with a consistent
methodology for determining both levels of service and
significant impacts.
It is recommended that the City Council also define as
"significant" an impact which causes an intersection on a
collector street operating at LOS A C 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 commission, or City Council on
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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.
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
criteria (Matrix)
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LUTM:CPD:
wjhcmsignun
COUNCIL MEETING: November 19, 1991
NUV 1 9 J991
Santa Monica, California
TO:
Mayor and City Council
FROM:
city staff
SUBJECT: Supplemental Staff Report to Agenda Item 9A
The following are staff1s responses to questions raised by city
Council during the November 12, 1991 study session on the
definition of a significant impact when analyzing traffic impacts
as part of the environmental review process.
1) tlci ty staff proposes to continue the existing method of
analyzing on an intersection by intersection basis. Instead,
we should be looking at traffic on a grid basis. This would
provide a more 'global' analysis of the situation and will
show how trips are diverted on to residential streets as
people avoid congested intersections."
Response: There are some programs currently being used to
analyze traffic on a grid basis, but they have been
at a more regional level, and their effectiveness has
yet to be determined. The methodology does not exist
for looking at a grid basis for the entire city.
Programs that exist only evaluate traffic on major
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ItJ.J -fo.9~ /I
NOV -1 9 1991
streets. They do not evaluate the impacts on
neighborhood or local streets.
Also, with the Master Environmental Assessment, the
city will be delivering to consultants background
information on future traffic levels. Through this,
the city can require consultants to present a chart
outlining where trips generated by a proposed
development are assigned. The city will then be able
to see where trips are distributed and make corrections
to account for likely redistribution of traffic impacts
on local streets.
2) If Why do we need to select a significance criteria now?1f
Response: The Civic Center Specific Plan E!R 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 criteria 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.
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3) liOn collector streets, LOS A, B or C intersections must go to
LOS D, E or F before any mitigation is required. This would
potentially 'give away' the entire capacity available at an
intersection to one development and then require mitigation
from all future developments impacting that intersection.
The best mitigation is prevention, and perhaps the City
should look at setting a percentage increase allowed at a
certain acceptable LOS intersection, with the closer you are
to capacity, the smaller the allowable increase."
Response: It may be very difficult to mitigate a LOS A, B or C
intersection since it is operating in a relatively
efficient manner. By setting a percentage increase, a
development that generates trips beyond that level may
have to perform mitigation which is not necessary or
desirable, like adding an unnecessary lane to an
acceptable intersection, or making unnecessary changes
to the intersection design to accommodate more traffic.
The question of IImitigatingll traffic impacts where no
real problem exists (by definition LOS A, Band C is
acceptable for collector streets, and LOS A, B, C and D
for arterials) really becomes a land use consideration.
Council must make a decision, do we want to somehow
limit or control development or address traffic
impacts?
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4) uWhy does the LOS F significance criteria use v/c? Isn't
that what CMA was based upon and we are trying to move away
from that?1I
Response: The seconds of delay criteria provided by HCM for
determining LOS calculations becomes invalid once you
reach LOS F intersections. For determining incremental
increases at an intersection due to project impacts,
the only quantifiable component to use under the HCM
methodology is the HeM provided vlc ratio when LOS F
has been reached.
Both HCM and CMA provide vlc ratios. HeM also provides
seconds of delay, which is used for calculating the LOS
of intersections. However, the HCM provided vlc is
different from the CMA v/c. The HCM vlc takes into
account many factors impacting an intersection,
including lane width, presence of bus stops, and
presence of parking near the intersection. Therefore,
the HCM vlc ratio is more detailed and reflective of
actual conditions than the CMA vlc ratio previously
used by the City.
5) UHow much traffic are we allowing ~y saying a LOS F
intersection has a significance criteria of .011"
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Response: It is impossible to state generally how many vehicles
could be added to each intersection before an increase
in the HeM v/c of .01 is reached because each
intersection has different operating conditions. For
an intersection operating at the most optimum level, it
is estimated that a maximum of approximately seven
vehicles could be added to critical movements.
However, it is unlikely that intersections in the City
will operate with this efficiency, and that the number
of vehicles added would be substantially less than
seven. It is more likely that the number would be two
or three vehicles to the critical movements.
This is a very conservative significance criteria, with
the only alternative being a criteria of no vehicles
allowed.
Prepared By: Paul Berlant, Director of LUTM
Suzanne Frick, Planning Manager
Ron Fuchiwaki, Parking and Traffic Engineer
Liz Casey, Associate Planner
Paul Casey, Transportation Planner
Land Use and Transportation Management Department
Program and Policy Development Division
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