Monday, July 26, 2021

New ramp meter alleviates traffic congestion, improves safety at SR 167 to northbound I-405

By Jordan Longacre

A recently installed ramp meter on State Route 167 where drivers merge onto northbound Interstate 405 in Renton will go live for the first time on Tuesday, July 27. Anyone who has merged onto northbound I-405 from SR 167 has likely been caught in heavy traffic or even experienced a collision at this location. The SR 167 ramp meter will help alleviate congestion and incidents and prevent free-flowing vehicles from hitting a dead-stop at the I-405 merge point, improving efficiency and safety for drivers.

The new ramp meter will regulate traffic during the peak morning commute from 6 a.m. to noon and intermittently during the afternoon in response to heavier traffic. This traffic change is part of an adaptive freeway metering system that automatically activates and adjusts each ramp meter based on current conditions. Additionally, our traffic engineers will monitor the meter timing and adjust as needed to ensure the best traffic flow.

Why do we meter?
Without ramp meters, vehicles entering the freeway merge together in tightly packed groups, which can cause drivers on the highway to slow down or even stop as they merge. This results in slower speeds, both on the freeway and on the ramp, quickly contributing stop-and-go conditions. By pacing the vehicles evenly at a controlled rate, the merge is much smoother.

In the case of SR 167 to I-405, vehicles currently go from traveling at 60 mph to a dead-stop near the merge point as they enter I-405. The meter prevents tightly spaced groups of cars from merging onto the freeway at once, which will keep I-405 flowing smoother during those times. The largest benefit of the new ramp meter will be at the beginning and the end of the peak commute times. By metering at the beginning of the commute, free flow on I-405 continues longer. Metering at the end of the commute ensures free flow returns faster, improving travel for all motorists.

How will a ramp meter be effective and efficient here?
Based on recent data collected along SR 167 and I-405, traffic along SR 167 is now exceeding 2017-2019 pre-pandemic averages and volumes along I-405 are within 10 percent of 2017-2019 pre-pandemic averages. With traffic on I-405 likely to continue increasing, the ramp meter will allow us to better manage the high traffic volumes entering I-405. If the meter can manage SR 167 now, as our analysis shows, it can manage pre-COVID volumes due to the similarity to the 2017-2019 midweek average.
  • Ramp meters automatically respond to changing traffic conditions. If traffic is flowing well, then the meter is not activated. During periods when traffic flow is beginning to slow down or conditions have become congested, ramp meters are in operation and automatically adjust to move vehicles more efficiently.
  • Ramp meters really show their effectiveness when it comes to managing traffic following a crash on the freeway, helping traffic recover quicker.
  • The ramp meter can help delay and/or prevent congestion from extending past the SR 167/I-405 merge point.
  • The SR 167 ramp meter can help alleviate and prevent free-flowing vehicles from hitting a dead-stop at the I-405 merge point.
  • A portion of the ramp shoulder area will serve as an additional lane during metering periods, preventing ramp metering backups onto SR 167.
  • Ramp meters are designed to help evenly pace vehicles merging onto the freeway, providing consistent gaps between vehicles, rather than multiple vehicles flooding onto the highway at once.
  • Ramp meters create a steadier flow of traffic onto the highway and reduce travel times.
A look at the direct connector HOV ramp that connects SR 167 to northbound I-405 in Renton.

How does this improve safety?
Ramp meters are an effective tool used to operate state highways safely. More than half of the collisions over the past five years have occurred where SR 167 merges onto northbound I-405.
  • Ramp meters improve safety by allowing one vehicle at a time to merge onto the interstate, reducing the severity and number of collisions due to merging traffic.
  • Gaps between merging vehicles will make the merge easier for drivers and reduce the risk of collisions.
What should I expect as a driver?
For drivers who normally use the SR 167 to I-405 ramp, one notable change will be the addition of a 1,250-foot shoulder lane that will also be metered. The ramp meter will have two signals that will alternate green lights to cycle vehicles through at an even pace. Signs are in place indicating both lanes are available when the ramp is metered.
  • Our traffic studies have shown drivers currently spend about 3 to 8 minutes on the SR 167 ramp to I-405. With the operation of the ramp meter, we expect morning commuters to experience similar delays on the ramp, while some evening commuters may experience a slight increase in ramp delays, but with the added benefit of safely merging into traffic flow on northbound I-405.
  • More consistent delays on the ramp for drivers will help provide better merging conditions, reducing overall congestion in the area.
The shoulder of the SR 167/I-405 ramp will be converted to a 1,250-foot storage lane (here on the far right) to be used for peak travel times to help keep traffic moving.

Will HOV and transit be able to use the ramp meter?
Carpools, vanpools and transit traffic will be able to use the I-405 Direct Connector HOV flyover ramp that connects SR 167 to I-405 via the leftmost travel lane but can also use the metered general-purpose ramp if they choose.
  • Metering the exit to I-405 would likely encourage eligible motorists to use the Direct Connector HOV flyover ramp.
  • The Direct Connector HOV flyover ramp adds additional capacity (HOV, transit, emergency vehicles) and reduces traffic volume that would have been using the normal ramp in previous conditions, again alleviating traffic congestion.
  • The general purpose and HOV lanes on northbound SR 167 just north of the South 180th Street interchange are excluded from the toll algorithm as the High Occupancy Toll Lane ends in this location.
  • We will be monitoring traffic on the Direct Connector before and after metering begins and will make recommendations regarding enforcement.