Roundabout Evaluation by zbm17245


									                                                                       Technical Memorandum
                                                                           September 6, 2007

Roundabout Evaluation

Additional analysis of the grade-separated roundabout interchange alternative at the west end
of the Sellwood Bridge has been completed by the CH2M HILL team. The analysis found that
the roundabout interchange would offer no substantial benefits over the signalized interchange,
but in many cases would actually perform worse. This memorandum outlines the additional
analysis and discussion of the roundabout interchange with respect to traffic operations,
pedestrian and bicycle circulation, transit operations, and freight considerations.

Traffic Operations
The potential traffic operations of the roundabout interchange concept were evaluated with
regard to the intersection’s overall vehicular delay and level-of-service (LOS), and the delay,
LOS, and vehicle queuing of each roundabout approach. The analysis was conducted using
preliminary year 2035 a.m. and p.m. peak hour traffic forecasts. SIDRA INTERSECTION
software, a popular program for single- and multi-lane roundabout analysis, was used to
estimate the intersection’s traffic performance.
The analysis assumed the roundabout lane configuration as shown in the attached illustration.
The analysis did not, however, account for any interaction or delays associated with pedestrians
or bicyclists crossing the roundabout at-grade. In addition, the analysis assumed that the
roundabout would function in isolation, i.e., it would not be influenced by traffic queues
resulting from bottlenecks occurring downstream of the roundabout.
According to the SIDRA results, during the weekday a.m. peak hour, the roundabout would
function at LOS “B” conditions. All turning movements would function at LOS B or better, with
the exception of the southbound-to-eastbound movement (southbound Highway 43 to
Sellwood Bridge) and the westbound-to-northbound movement (cemetery to northbound
Highway 43), which would each operate at LOS C.
During the weekday p.m. peak hour, SIDRA estimates that the intersection would operate at
LOS C conditions, but all movements from the northern leg (southbound approach) and
southern leg (northbound approach) would function at LOS D. It was estimated that motorists
approaching the roundabout from the north and the south would face an average delay ranging
from 37 to 45 seconds, with the southbound 95th percentile vehicle queue extending about 750
feet and the northbound 95th percentile queue extending about 260 feet.

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The above results did not take pedestrian and bicycle movements within or across the
roundabout into consideration. Inclusion of these movements would result in worsened
operations than discussed.
In addition, the operational analysis did not consider how downstream bottlenecks, causing
vehicular queues to the west end interchange, could affect the roundabout’s operation. Over-
capacity traffic conditions are expected to continue to result along Tacoma Street east of the
Sellwood Bridge, causing vehicle queues that extend to and through the roundabout during the
weekday p.m. peak period and during other periods. According to Federal Highway
Administration (FHWA) roundabout guidelines, as documented in Roundabout Guidelines: An
Informational Guide (June 2000):
           Some conditions may preclude a roundabout at a specific location. Certain site-related
           factors may significantly influence the design and require a more detailed investigation of
           some aspects of the design or operation, (for example) proximity of bottlenecks that would
           routinely back up traffic into the roundabout, such as over-capacity signals, freeway
           ramps, etc. The successful operation of a roundabout depends on unimpeded flow on the
           circulatory roadway. If traffic on the circulatory roadway comes to a halt, momentary
           intersection gridlock can occur. In comparison, other control types may continue to serve
           some movements under these conditions.
In other words, recurrent eastbound vehicular queues extending along Tacoma Street and
across the Sellwood Bridge would effectively shut down most or all movements, from most or
all directions, through the roundabout interchange alternative. Conversely, several movements
could be served by the signalized interchange alternative under these conditions.

Pedestrian and Bicyclist Circulation
A meeting was conducted on August 14, 2007, with the Sellwood Bicycle/Pedestrian Working
Group to review potential pedestrian and bicycle issues related to the roundabout interchange
concept. The working group expressed their concerns about a roundabout, citing the direct
conflicts that would exist between nonmotorized users and motorized vehicles. Their
recommendation is that the roundabout be removed from further consideration.
However, if the roundabout interchange concept remains under consideration, the working
group identified the following potential strategies for accommodating bicycle/pedestrian safety
and convenience:

•     Provide grade-separated ramps linking the Sellwood Bridge with the Riverview
      Cemetery access road: Grade-separated ramps would enable nonmotorized users to travel
      between the bridge and Riverview Cemetery without an at-grade crossing of the vehicle
      travel lanes. Although this treatment would not address north-south bicycle/pedestrian
      movements through the interchange area, safety and convenience of east-west crossings
      would be enhanced.
•     Provide protected at-grade crossings: The roundabout concept could include protected at-
      grade crossings to facilitate safe and comfortable bicycle/pedestrian crossing movements.
      Potential treatments include an innovative signalized crossing such as the High-Intensity
      Activated Crosswalk (HAWK) signal, which stops approaching vehicles only when
      activated by a bicyclist or pedestrian. Protected at-grade crossings are especially important

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    where nonmotorized users wish to cross travel lanes with high vehicle speeds or with few
    gaps in the traffic stream.
•   Limit east-west bicycle movements to the roundabout’s south side: The highest vehicle
    volumes would occur on the westbound-to-northbound and southbound-to-eastbound
    movements. Conflicts with these heavy volumes could be avoided by limiting
    bicycle/pedestrian crossings to the roundabout’s south and west sides (and restricting
    crossings on the north side). Although this strategy would address bicycle/pedestrian
    crossings at locations with the highest vehicle volumes, nonmotorized users would still
    directly conflict with motorists entering and leaving Highway 43 from the roundabout’s
    south side. This strategy would also create out-of-direction travel for bicyclists/pedestrians
    approaching the interchange area from the north side of the Sellwood Bridge or cemetery
    access road by forcing them to divert to the other side of the roundabout.

Transit Operations
A meeting was conducted on August 14, 2007, with representatives from TriMet to review
potential transit needs with the roundabout interchange concept. TriMet’s main issues with any
interchange type include the following:

•   Ability to provide bus-to-bus transfer stops between Portland/Lake Oswego and
    Portland/Sellwood routes. Ideally, these stops would be located all at the same grade. This
    would require Highway 43 bus routes to exit at the interchange to connect to stops on the
    overpass and then merge back onto Highway 43.
•   Ability to provide bus-to-trolley transfer stops when and if the Lake Oswego trolley begins
While both interchange types could be designed to accommodate stops for all route directions,
the roundabout interchange presents more of a challenge in providing bus-to-bus and bus-to-
trolley transfers. One of the suggestions by the Bicycle/Pedestrian Working Group, if the
roundabout interchange alternative is pursued, would be to eliminate any crosswalks on the
north side of the roundabout. Under this provision, riders transferring from the
Portland/Sellwood bus route (westbound-to-northbound) to other route stops located in the
other quadrants of the interchange would have to follow a circuitous path, using the
switchbacks and the Willamette Greenway Trail to access marked crosswalks on the south side
of the roundabout. If the crosswalks on the north side were not eliminated, transferring riders
would be crossing the heaviest vehicle movements (southbound-to-eastbound and westbound-
to-northbound) of the intersection, which also caused concern to the TriMet representatives.
TriMet also was concerned about two other aspects of transit operations with the roundabout

•   Recurrent interchange congestion: As stated previously (see “Traffic Operations”),
    eastbound traffic congestion along Tacoma Street would continue to result in vehicular
    back-ups along the Sellwood Bridge. The back-ups, particularly during the afternoon
    commute period, as well as on weekends, would back up traffic into the roundabout,
    impeding transit movement in all directions through the intersection. It should also be
    noted that a roundabout offers no opportunity to provide a signalized queue jumper lane
    for buses.

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•     Future streetcar on bridge: If a streetcar route is added on the bridge in the future, the rail
      alignment would either pass through the central island of the roundabout to make the turn
      movement from Highway 43 to the bridge, requiring signalization of the roundabout, or the
      alignment would follow the lanes within the roundabout, potentially resulting in streetcar
      off-tracking issues (i.e., either the front end, the back end, or both, extending into an
      adjacent roundabout travel lane).

Freight Considerations
The Sellwood Freight Working Group met on August 14, 2007, to review the criteria scoring of
all 124 concepts, including the grade-separated roundabout interchange alternatives. During the
discussion of west-end interchange options, the working group reached a consensus that the
roundabout option should be eliminated from further consideration. The group cited an
increased risk of truck off-tracking accidents as a significant issue in their decision.
The working group understood that trucks are allowed to occupy both travel lanes when
circulating in a two-lane roundabout. However, in their opinion, this “rule” may not be known
by all users of roundabouts since roundabouts are a relatively new intersection type in Oregon.
Even if truck drivers are legally right in occupying both lanes of a roundabout, the group’s
opinion was that truck operators would be at risk of insurance and legal fees resulting from
involvement in off-tracking accidents.

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