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Bottlenecks and Chokepoints

Traffic Congestion

In Washington State, the growth in travel demand has outpaced expansion of transportation system capacity. Additionally there is little evidence that major levels of new investment in highway system capacity will be forthcoming, leaving the state with a backlog of capacity needs now and in the future. This imbalance of demand and capacity occurs in virtually every mode of transportation - at our airports, on our rail lines, and especially on our roadway systems.

The growing demand/capacity imbalance affects citizens’ daily lives and almost every sector of economic activity. Commutes to work are time-consuming and often aggravating. Non-work trips, too, must be planned to avoid congestion or with an extra time allowance to account for the lack of reliability in travel times. Freight delivery becomes slower and less reliable. Air pollution is exacerbated by cars and trucks stuck in traffic. Even rural areas that have never seen traffic jams are penalized when highway congestion associated with urban areas interferes with their agricultural products reaching ports and customers.

Washington is Growing

Population and jobs are expected to continue to grow in Washington State.
This population growth will translate into substantial increases in travel demand.
Washington’s workforce is also growing and will continue to a projected
3.9 million by the year 2030. This growth is leading to more travel and compounding delay.

Delay Occurs Mostly in Urban Areas

There is a projected growth in travel that will be concentrated in Puget Sound, Spokane and Vancouver. Consequently, the gap between demand and capacity will grow wider in the future, especially in the major urban areas and high traffic volume corridors.

Delay is more prevalent in urban areas with the greatest delay found in the Central Puget Sound area. The total delay across the state is estimated to be over 365,000 hours per weekday and represents about $1.6 billion annually in lost time.

Congestion Actually Reduces Capacity

Congestion in the form of vehicle delay creates inefficiency and has the effect of reducing freeway capacity. The graph below illustrates that although congestion increases and freeway speeds drop below the posted speed limit, the total throughput of the freeway increases until a maximum throughput is reached at about 45 mph. If congestion worsens beyond this point speeds and total throughput will drop rapidly. To optimize the efficiency of the freeway system we need to keep the traffic flow on top of the curve.

Congestion reduces the capacity of roadways by up to 50%. This efficiency loss can be seen more clearly in this graph. On a section of I-405 during the morning commute the throughput lost due to congestion was equal to nearly half the highway’s capacity. In other words, at the very time when the capacity is most needed the equivalent of one whole lane (out of two general purpose lanes) is lost to congestion. These efficiency losses often occur at bottleneck and chokepoint locations, which can severely hinder the entire system’s performance.

Bottlenecks and Chokepoints are Major Causes of Delay

Bottlenecks and chokepoints are typically locations on the system where geometry and traffic patterns contribute to congestion. Examples include the Kirkland crawl on I-405, the Southcenter hill climb on I-5, SR-18 between I-5 at Federal Way and SR-167 at Auburn, the Renton S-curves on I-405, US 2 near Monroe, and interchanges such as I-5/I-90 in Seattle, I-405/I-90 in Bellevue, and I-5/SR 16 in Tacoma.

In addition, weather can cause congestion or affect the passability of a roadway creating a bottleneck or chokepoint. Avalanche control on the I-90 Snoqualmie Pass and roadways closed due to spring thaw restrictions are examples of weather related bottlenecks and chokepoints throughout the state.

Operational and Targeted Capital
Investments Can improve Roadway Productivity

The state currently manages a number of programs to improve the productivity of our highway system. These programs include operational measures and capital investments.

Washington State is considered a leader in the use of high occupancy vehicle (HOV) lanes, ramp metering, and signal synchronization to improve the maximum throughput of a roadway experiencing congestion.

HOV Lanes

HOV lanes increase the efficiency of our system in three ways: by limiting the number of vehicles, overcrowding of the lane is prevented and vehicle throughput is increased, while the higher occupancy rate increases person-throughput and creates an incentive to commute via HOV modes.

Fixing Bottlenecks and Chokepoints

Targeted traffic flow improvements can also make a significant difference in system performance. The recently completed I-405/SR 167 Flyover ramp is a good example of one such targeted investment. Prior to the opening of the new ramp stop-and-go conditions occurred weekday mornings between 6:45 and 8:00 a.m. Immediately after the opening of the new ramp, the stop-and go condition was almost entirely eliminated. In the past year we’ve seen continued growth in the I-405 mainline volumes as well as the I-405 southbound to SR 167 southbound ramp. While serving higher volumes, the congestion at the interchange area is still considerably lower than the conditions prior to the project. On weekends, both the stop and go traffic and heavy congestion conditions have been essentially eliminated.

Ramp Meters

The effect of ramp metering in reducing delay is well documented. Prior to ramp metering SR 520, stop-and-go conditions occurred between 7:25 and 9:25 am. After the ramp metering, most of the stop-and-go condition was eliminated.