HomeMy WebLinkAboutTraffic Study Requirement Staff Memo
TO: Board of County Commissioners
FROM: Peter Russell, Senior Transportation Planner
DATE: January 28, 2014
MEETING: February 24, 2014
SUBJECT: Work session with staff on proposal to move traffic study requirements from Title
17 to Title 18 and revise the level of service (LOS) standard from LOS C to LOS
D for new County facilities (TA-13-2)
BACKGROUND
The County first added a traffic study requirement into the development code in 2006, placing it
in Chapter 17.16, which deals with master plans and subdivisions. The traffic study
requirements are triggered by any use that will generate more than 50 new weekday trips;
however, over the years the County has accepted numerous land use applications that were not
subdivisions or master plans, yet easily surpassed that trip threshold and thus had to submit a
traffic analysis. At a Jan. 9, 2014, public hearing the Planning Commission recommended the
Board of County Commissioners (BOCC) approve the changes in TA-13-2.
PROPOSAL
A more logical location for the traffic study requirement would be DCC 18.116, Supplemental
Provisions, which applies to all land use zones and already includes many general
transportation items such as standards for Class I and II road projects, clear zones, off -street
parking, and bicycle parking. TA-13-2 would move the traffic study requirements now found in
DCC 17.16.115 and shift them to DCC 18.116 while creating a Section 18.116.310, Traffic
Study Requirements. The cross-references in DCC 18.124.080, Site Plans, would be modified
from DCC 17.16.115 to DCC 18.116.310 and DCC 17.16.115 would also add a cross-reference
to the new DCC 18.116.310.
The second aspect of TA-13-2 is to change the performance standard from Level of Service
(LOS) C for new roads and intersections to LOS D, which is the standard for existing County
facilities. By having LOS D as the performance standard for all existing and future County road
segments and intersections, the resulting road network will be orderly and cost-effective.
The proposed change from LOS C to LOS D for new County facilities would affect a minimal
number of future projects. The County’s Transportation System Plan (TSP) at Table 5.3.T1
(County Road and Highway Projects) lists 53 total improvements to County roads. Of those 53
County road projects only five are future roads or intersections. Of those five, four are low
priority (11-20 years) and one is high priority (1-5 years). None of the five is funded.
2
Staff has provided summary tables based on the “Highway Capacity Manual” (HCM) for the
relevant LOS standards. The County’s LOS standards apply only to County roads; when
County roads intersect State highways the performance standard is set by the “Oregon Highway
Plan” (OHP).
The LOS for intersections is based on delay. Table 1 compares the difference between LOS C
and LOS D at both unsignalized intersection and signalized intersections. The comparison
provides a sense of scale of the proposed change in terms of County road operations.
Table 1
Average Seconds Delay per Vehicle by Level of Service and Intersection Type
Level of Service Unsignalized Signalized
LOS C 15-25 seconds 20-35 seconds
LOS D 25-35 seconds 35-55 seconds
Worst case delay (minimum LOS
C subtracted from maximum LOS
D) 20 seconds 35 seconds
LOS applies not just to intersections, but also to lengths of roadway. The major factors for LOS
along rural segments are traffic volumes, how easy it is to pass (which is indicated by percent
time following a vehicle), and travel speed. The LOS continuum remains the same. LOS A has
free flow conditions at or above the posted speed. At the opposite end LOS F has stop and go
traffic, high volumes, and very variable speeds. A two-lane rural highway on level terrain can
accommodate 3,200 passenger cars per hour (pc/h) in both directions. The capacity of a single
travel lane is 1,700 pc/h. Table 2 compares the potential differences between LOS C and D and
the pc/h is for traffic in both directions. As pc/h increases so does percent time following while
travel speed declines. In other words, more traffic means fewer opportunities to pass.
Table 2
Level of Service, Capacity, Percent Time Following, and Speed for Two-Lane Rural Roads
Level of Service Passenger Cars/Hr Pct. Time Following Free Flow Speed
LOS C 1,190 pc/h 50-65 45-50 mph
LOS D 1,830 pc/h 66-80 40-45 mph
Worst-Case Difference +640 pc/h +30 -10 mph
In terms of rural, two-lane roads at LOS C, traffic volumes have increased and platoons (lines of
several vehicles in a row) can begin to form and passing becomes more difficult. Speeds still
remain above 45 mph. Traffic flow remains stable, although congestion can occur in the
presence of slow-moving vehicles or lots of turning vehicles.
LOS D on a rural, two-lane road means traffic begins to be unstable and passing opportunities
become extremely difficult while demand for passing increases. Platoons of five to 10 vehicles
become common. Slow-moving vehicles and turning vehicles have an even greater adverse
effect.
The County’s TSP on pages 125-126 found the only segments that exceed LOS D in 2030 were
on the urban margins of Bend, Redmond, and La Pine.
Staff has not yet set a public hearing date on proposed Ordinance 2014-001, which will
implement TA-13-2.