Congestion Monitoring
Report Summary
2005

Congestion Management Network

The CMS corridors consist of regional facilities that are currently or are likely to become congested. They form a subset of the Vancouver/Clark County Regional Transportation System. The congestion management network is made up of thirty corridors. The endpoints for each corridor represent locations where the characteristics of the corridor change significantly. 

A corridor approach that incorporates parallel routes and transit services provides a regional orientation and responds to the multimodal and alternative travel themes of the Federal Transportation Act. Although group by corridors, data is reported for individual facilities.

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Corridor Congestion Ratio

The corridor congestion ratio is an aggregation of the volume/capacity (V/C) ratios for the individual segments that make up a facility within a corridor. For each segment in a corridor, the V/C ratio, vehicle miles traveled (VMT), and VMT-weighted V/C ratio (the product of the V/C ratio and VMT) for the peak hour are calculated. The CCR is the sum of the weighted link ratios. It is intended to show the overall V/C ratio for the length of a corridor instead of a single location on the facility. 

The PM period shows congestion along major facilities such I-5 South, I-205 South, SR-14, SR-503 South, Fourth Plain East, and Burton Road. Recently completed improvements along Burton Road are likely to reduce the corridor congestion ratio in that corridor.

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Corridor Speed as Percent of Speed Limit

The City of Vancouver and Clark County collect PM peak period travel time for concurrency purposes along concurrency corridors. RTC has contracted to collect AM peak and additional PM peak travel time data in corridors not covered by the concurrency data collection effort. Travel speed is computed from the travel time data. It consists of utilizing the travel time and distance to calculate an average travel speed. Speed was then converted to a percent of posted speed for each of the congestion management corridors.

The speed percentage for the freeway facilities are generally close to the posted speed limit. While facilities with multiple signalized intersections are generally between 65% and 80% of the posted speed limit. While speed alone is not an indicator of congestion, higher speed facilities are more attractive and generally carry more vehicles.

In the PM period, I-5 South, SR-500, Andresen, Mill Plain East, 112th Avenue, 137th Avenue, 134th Street, Main Street, and 18th Street operate at less than 65% of the posted speed.

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Areas of Concern: Volume to Capacity Ratio

Each facility within a corridor is further divided into a series of segments. A segment is the portion of roadway between major intersections or interchanges. For example, between 78th Street and 99th Street represents a segment on each facility in the I-5 Central Corridor. Using segmental data, areas of concern were identified.

Volume to capacity ratio areas of concern are segments within individual corridors that have a volume to capacity ratio greater than 0.9. This limitation on road capacity leads to congestion. To the most part, solutions to these concerns have been identified in local plans. However, many of these solutions are expensive and are not planned within the next few years.

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Areas of Concern: Travel Speed

A travel speed lower than 60% of the posted speed limit is an indicator of delay, which can result in congestion. As development occurs along a corridor, travel speed often decreases because of multiple driveways and additional traffic signals. One of the difficulties of maintaining arterials is balancing access to land uses and maintaining the throughput travel speed of the corridor. Many of the solutions to these concerns are not programmed in plans, but are potentially low cost solutions such as access management, traffic signal coordination, and signal timing.

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This report was prepared by: