SR-506-003-04 (15)
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MAY 1 4 1991
LUTM:PB:RF:db:losreprt:desk:genserv Santa Monica, California
council Meeting, May 14, 1991
To: Mayor and City Council
From: city staff
subject: Recommendation to Designate a Methodology to be Used
in Traffic Studies to Calculate Level of Service
Introduction
This report recommends the City council designate a methodology
to be used in traffic studies to calculate level of service.
Discussion
At the April 23rd study session the city council was presented
with information about the methodologies that are currently being
used to calculate the traffic level of service at intersections
in conjunction with traffic studies. Level of service (LOS) is a
qualitative measure that is used to describe the condition of
traffic flow at an intersection. The level of service can range
from A, representing free flow conditions, to F, representing an
intersection substantially at capacity.
During the study session the City council was presented with
information about the three methodologies that are currently
being used throughout Southern California to calculate level of
service: the Critical Movement Analysis (CMA) methodology, the
Highway Capacity Manual (HeM) delay methodology and the
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Intersection Capacity Utilization ( lCU) methodology. The City
currently requires the use of the CMA methodology for traffic
studies of proposed projects in the city.
As was indicated at the study session, the methodology of
calculating level of service is also being examined by the Los
Angeles County Transportation Commission (LACTC) Congestion
Management Agency. As a result of the passage of Proposition
111, every urban county in the state is required to develop a
Congestion Management Plan. Part of the Los Angeles County plan
includes an examination of level of service standards. It is
necessary for the city's choice of methodology to be compatible
with the methodology adopted by the LACTC. However, it is likely
that any of the three methodologies being considered by the City
will be compatible with the Congestion Management Plan.
On April 24, 1991, the consultant retained by the LACTC to review
the issue of level of service, recommended the LACTC adopt the
ICU methodology for the first year and the HCM Planning
methodology for subsequent years. The HCM Planning methodology
is not the same methodology discussed at the council study
session. It is a very simple study giving a qualitative answer.
However, at this time, the LACTC has not taken any action on the
consultant's recommendation. A draft plan is expected mid May
which will reflect the LACTC staff's recommendation on this and
other subjects.
The following is a summary of each methodology: the HeM method of
intersection capacity analysis considers the details of each
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approach and each separate movement within each approach. It
calculates the per vehicle delay for each movement and assigns a
level of service to each. It then considers all approaches and
movements and provides a summary of the level of service for the
entire intersection. This method requires detailed information
on all prevailing traffic, roadway, and signalization conditions
at the intersection. Data requirements include traffic volumes
by vehicle type, specific geometric conditions including street
and lane widths, parking characteristics including permitted
areas and use patterns, pedestrian activity, and details of
signal timing. It also requires the user to identify a
saturation flow rate for each approach of the intersection on a
per lane basis. The output from this method is an average delay
per vehicle and level of service rating for each approach and for
the intersection as a whole.
The CMA method is a procedure that incorporates the effects of
geometry and traffic signal operation and results in a volume to
capacity ratio used to determine level of service for the
intersection as a whole operating unit. The analytical base for
this method is the understanding that each signalized
intersection has a combination of conflicting movements which
must be accommodated. The output from this method is a
volume/capacity (v/c) ratio and a level of service for the
intersection as a whole.
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The reu uses the same principles as the CMA. The primary
difference is that the leu develops a v/c ratio for each separate
movement, identifies those that are critical, and then adds them.
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As indicated, the CMA identifies the critical movements, adds
them and then calculates the v/c for the entire intersection.
All three methodologies are accepted throughout Southern
California and probably the nation. The CMA and ICU methods are
similar and are based on a comparison of the actual traffic
volume at an intersection and the intersection's theoretical
traffic volume capacity during an hour. The HCM method involves a
more detailed calculation and equates a level of service to a
delay per vehicle.
Additional Traffic Study Methodoloqy Issues
At various city/community meetings, members of the public have
also questioned other aspects of traffic analysis done by the
city. One such issue relates to which primary source for trip
generation rates should be used for proposed projects. Trip
generation rates are average values which are used to estimate
the number of traffic trips which would be generated by a
proposed use or development. The city currently requires the use
of the most current volume of the Institute of Transportation
Engineers (ITE) Trip Generation Manual. This manual is
extensively used by professionals throughout the country.
Additional sources which have similar information are the San
Diego Association of Governments (SANDAG) Trip Generation Manual
and other trip generation studies which have been performed by
other agencies or consultants. As indicated at the study
session, the most precise for trip rates is to develop trip rates
specifically for Santa Monica. However, this is an extremely
expensive and cumbersome process and is not practical at this
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t.ime. since the city does not have the resources to conduct its
own set of trip generation studies nor do we believe that the
results of such studies would be of significant difference from
the ITE values, staff believes the best and most thorough source
of information is the ITE Trip Generation Manual.
Another issue which has recently been discussed is the
methodology of how to count traffic for traffic studies. For
example, for an afternoon peak hour traffic count, the current
practice calls for the intersection to be counted for a two-hour
period during the afternoon peak period (generally 4:00 pm to
6:00 pm) with the numbers of cars traveling through the
intersection recorded every 15 minutes. Typically these counts
are taken on a Tuesday, Wednesday or Thursday to capture a
"typicalll average weekday period. When the actual counting is
completed, the four highest, consecutive 15 minute periods are
chosen for the peak hour. This peak hour number is then used to
calculate the intersection's level of service.
Alternate counting methods are available and have recently been
discussed in public forums. One alternative method includes
counting an intersection in a similar manner, but counting for
five consecutive weekdays and then averaging all of the data for
the five day period to develop one peak hour nUInber. Another
method counts an intersection for 15 minutes and then multiplies
the count by four to develop a peak hour number.
The method the City currently uses is the standard methodology
for counting used by most traffic engineers. The five-day
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averaging methodology is available, but is burdensome and costly
and may not provide any more accurate information. The 15-minute
method often provides false, inaccurate information.
Related Issues
During the study session, questions arose as to the impact the
Council designation would have on traffic study costs, the time
required to complete a traffic study, and the progress of the
civic Center Specific Plan and Pier EIRs. If the Council chooses
to designate the CMA or lCU methodologies, there will be no
impact on traffic study costs or time. If the Council chooses to
designate the HCM methodology, it is estimated the costs of
traffic studies will increase by 30 to 50 percent and the length
of time required to complete a study will increase by one to two
months.
with regard to the council's action on methodology and the impact
on the Civic Center and Pier EIRs, unless otherwise directed by
the Council, staff plans to continue progress on these studies.
These reports are currently in the staff review stage and should
be presented as a draft EIR soon. The Council should note the
traffic study portion of these EIRs is using the CMA methodology
and other practices previously used for traffic studies. If the
Council chooses to have these studies use another methodology,
the Council should direct staff to do so in conjunction with the
designation of level of service methodology with the under-
standing this will result in a significant delay of these
reports, perhaps as much as one year and added costs.
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Moratorium Work proqram
If the City Council chooses to designate the HCM methodology, it
will be necessary to extend the time period for the MEA and the
overall Moratorium work program by at least two months. This
additional time is necessary to obtain the additional information
needed to conduct the HCM calculations.
In addition, during the Planning commission discussion on the
MEA, some Commssioners asked to include summer counts in the
evaluation. At that time, staff indicated summer counts would
not fit into the present time constraints of the Moratorium work
program. However, the Commission believed it was important to
include this information. Therefore, if the Council agrees it is
appropriate to include summer counts, it will be necessary for
the Council to extend the Moratorium work program by an addi-
tional month.
In summary, if the Council chooses the HCM methodolgy and chooses
to include summer counts, the Moratorium work program must be
extended by a total of three (3) months.
Summary
The following is a summary of the potential actions and
information the Council should consider.
L The Council should designate a methodology to calculate level
of service.
2. The Council should decide whether to act on the issues of
trip generation and traffic counts.
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3. If the Council chooses the CMA or the lCU methodologies the
cost and time to conduct traffic studies will not change from
current practices.
4. If the council chooses the HCM methodology the cost of
traffic studies will increase by 30 to 50 percent and the
time required to complete a study will increase by one to
two months.
5. The Pier and civic Center EIRs will proceed as planned, in-
cluding using the CMA methodology, unless otherwise directed
by the Council.
6. If the Council chooses the HCM methodology, the Council must
extend the Moratorium work program by two months.
7. If the Council chooses to include summer weekday traffic in-
formation in the MEA, the Council must extend the Moratorium
work program by one month.
Recommendation
It is recommended the City Council designate a methodology to be
used on all traffic studies to calculate level of service at
intersections. In reviewing the information presented on the
three methodologies, staff believes that all three are acceptable
for the purpose of doing project specific traffic impact studies.
However, if the Council believes the City should start collecting
the more detailed data, presented in terms of delay of travel and
other impediments to travel rather than the ratio of volume to
capacity derived from the CMA and lCU methods, and the Council
believes such data is worth the added time and cost associated
with the HCM method, then staff recommends that the City Council
select the HCM methodology for traffic studies.
If the City council designates the HCM methodology, it is also
recommended the Moratorium work program be extended for two
months. staff also recommends that the City Council choose to
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include summer weekday counts in the MEA and therefore extend the
Moratorium work program one additional month. The total
additional time for the HCM and summer counts would thus be three
months.
With regard to any other related traffic study issues, such as
trip generation rates or traffic counting methodologies, staff
recommends that the Council take no action and leave these
technical study issues to the City Parking and Traffic Engineer's
best professional judgement.
Prepared by: Paul Berlant, Director of Land Use and
Transportation Management
Ron Fuchiwaki, city Parking & Traffic Engineer
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