SR-SS-1 (9)
. . ss... ,
.R 2 3 j:n,l
LUTM:PB:RF:db:
April 23, 1991
INFORMATION ITEM
To: Mayor and city council
From: City staff
Subject: study Session about Methodologies to Calculate
Level of Service for all Traffic Studies in
eity of Santa Monica
Introduction
This report provides information to the city council about a
scheduled April 23rd study session about methodologies to
calculate the level of services for all traffic stud.1.es in the
city of Santa Monica.
Discussion
For several months, members of the City Council, members of the
Planning Commission and staff have discussed the methodology that
is 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.
Similar to grades in school, A represents excellent conditions,
and F represents failing conditions.
- 1 - $5-1
If'R 2 3 1991
'-.?? . .
A ~~ ~~"
T e me odology 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.
Durlng the study session, scheduled for April 23, 1991, the Clty
council will be 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 (HCM)
delay methodology and the Intersection Capacity Utilization (reU)
methodology. The City currently requires the use of the CMA
methodology for traffic studies of proposed projects in the City.
The HCM method of intersection capacity analysis considers the
details of each approach and each separate movement within each
approach. It calculates the per vehicle delay f~r 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
\ ' ...,
. .,.'" ,.;....: - 2 -
. e
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. This flow rate
assumes ideal conditions. The method is "primarily intended for
use in the analysis of existing conditions, or for future
conditions where the traffic volumes, mix of traffic 1 geometric
standards, and signal control parameters are established through
projections and trial designs" (19B5 Highway Capacity Manual) .
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. Regardless of the complexity of the
intersection and its traffic signal operations, the critical
volumes (when placed on a per lane basis) cannot physically be
accommodated beyond the 2000 passenger cars per hour of green
(pchg) limit, and in practice cannot be accommodated beyond about
1500 to 1800 pchg. The upper limit of this practical limit (Le. ,
1800 vph) assumes ideal traffic conditions, i.e., no trucks, no
buses, 12-foot lanes, no conflicting left turns. Factors have
been developed which allow modification of the capacity value of
- 3 -
. .
1800 pchg to consider the lane width, buses and trucks, bus stop
operations, left-turns, right-turns and pedestrians, parking
activity, peaking characteristics, and number of signal phases.
At a typical intersection, the combined effect of these factors
results in an average capacity of 1500 pchg for a two-phase
signal, a value which is used in the critical Movement Analysis
method as described in Circular 212. It is also the value that
has been typically used in the city of Santa Monica. The output
from this method is a volume/capacity (v /c) ratio and a level of
service for the intersection as a whole.
The ICU uses the same principals as the CMA. The primary
difference is that the rcu develops a v/c ratio for each separate
movement, identifies those that are critical, and then adds them.
As indicated, the CMA identifies the critical movements, adds
them and then calculated the vie for the entire intersection.
All three methodologies are accepted throughout Southern
Californl.a and probably the natl.on. In summary, the CMA and reu
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. Upon hearing the presentation
in the planned study session, Council will have been given a
comparison of the methodologies and benefit of each.
- 4 -
--
. .
Additional 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 lS
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.
Finally, 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
"typical" 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.
- 5 -
~- -- - --~ -------
. . .
Alternate counting methods are available and have recently been
discussed in public forums. One alternative method includes
counting an intersection l.n a siml.lar manner, but counting for
five consecutive weekdays and then averaging all of the data for
the five day period to develop one peak hour number. Another
method counts an intersection for lS 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 traffic for traffic studies used by most traffic
engineers. The five-day averaging methodology is available, but
is burdensome and costly and may not provide any more accurate
information. The IS-minute method often provides false
inaccurate 1nformation.
Conclusion
The discussion and public debate over the study methodology needs
to be resolved by Council action. The three methodologies
described above all meet different peoples' expectations. There
is little reason to not accept one methodology over another
except as relates to the Congestion Management Plan. Santa
Monica should adopt a methodology consistent with that selected
in the Congestion Management Plan.
with regard to the other issues, trip generation rates and
traffic counting, staff sees no reason to alter the system
currently ln place. The ITE manual is a nationally accepted
primary source which has been designed to meet urban traffic
analyses needs. The counting method currently used by the City
- 6 -
- e
.
is the one used most often by the profession and represents the
best data for a reasonable cost.
The City Council will be asked to adopt a standard methodology to
calculate level of service at its May 14th meeting.
Prepared by: Paul Berlant, Director Land Use and Transporation
Management
Ron Fuchiwaki, eity Parking & Traffic Engineer
(los)
- 7 -
. .
NOTICE OF SPECIAL MEETING OF
SANTA MONICA CITY COUNCIL
APRIL 23, 1991
NOTICE IS HEREBY GIVEN that a Special Meeting of the City Council
will be held at 6:00 P.M., on Tuesday, April 23, 1991, in the
Council Chambers, 1685 Main Street, Santa Monica for the purpose
of conducting the following business:
STUDY SESSION:
SS-l: Traffic study Methodology.
No other business will be conducted at the Special Meeting.
Thereafter, the City Council will convene in Regular Session.
1~
Judy Ab~t
Mayor, City council,
Chairperson, Agency and Authority
ACKNOWLEDGEMENT OF RECEIPT:
Ken Genser Kelly Olsen
Mayor Pro Tempore councilmember
Chairperson Pro Tempore Agency and Authority Member
Robert T. Holbrook Antonio vazquez
councilmember councilmember,
Agency and Authority Member Agency and Authority Member
Herbert Katz Dennis Zane
Councilmember Councilmember,
Agency and Authority Member Agency and Authority Member
NOTICES
- 1 -