Sammamish asserts that traffic delays at major commuting intersections are much shorter than Issaquah and Washington State report. It appears that the Sammamish consultants are not accurately stating traffic conditions.
The city of Sammamish is embarking on a major change in direction for future development. A key element under current lively discussion is the revised Town Center Plan. Because it will have negative environmental impacts, the City is in the midst of a Supplemental Environmental Impact Statement (SEIS). Traffic is one of the two main problems addressed in the draft SEIS.
Sammamish uses intersection delay as its only measure of traffic level of service (LOS). The traffic analysis in the draft SEIS includes very large tables comparing peak hour intersection delays for two alternatives to one tenth of a second at more than 60 intersections 20 years in the future! The DSEIS states “According to the analysis results, there are no new future deficiencies caused by the Sammamish Town Center project.” This despite a 10 – 20% increase in the number of peak commute drivers.
Before accepting these 20 year predictions as fact, it would be good to examine whether the current estimates are in fact correct. Intersection delay is computed, not actually measured. The computations are complex. Should we believe the results? We can compare the Sammamish estimates with those of other jurisdictions for the same intersections.
I90 commuters
Almost all of the Sammamish residents who commute using I90 pass through one of two key intersections that are in Issaquah. Both Sammamish and Issaquah calculate delay at these intersections. The results do not agree.
The roundabout at East Lake Sammamish Parkway and SE 43rd Street is one of the most important intersections to Sammamish drivers. According to city traffic counts, it can have average daily traffic of more than 25,000 vehicles.
The stop light intersection of Issaquah Pine Lake Road and Issaquah Fall City Road is the second most used for Sammamish I90 commuters. According to city traffic counts, it can have average daily traffic of more than 23,000 vehicles.
PM (5-6) Peak Hour LOS
| Intersection | Sammamish | Issaquah |
|---|---|---|
| East Lake Sammamish Parkway and SE 43rd St | A | D |
| Issaquah Pine Lake Rd and Issaquah Fall City Rd | C | E |
Issaquah states that the roundabout delay is approximately 5x longer than Sammamish does. Issaquah states that the stop light delay is approximately 3x longer than Sammamish does. (Issaquah provides only a PM Hour LOS score.)
For a roundabout
LOS score A. Very low delay, free flow. Drivers
experience little or no delay when entering.
LOS score D. Approaching unstable flow. Significant
delays and frequent queues.
For a stop light
LOS score C. Stable flow with acceptable delay.
Progression is fair, and cycle lengths may be moderate. An occasional
approach phase may be fully utilized.
LOS score E. Unstable flow. High delay. The
intersection operates at or near its capacity. Long queues and frequent
cycle failures are common.
Who is right?
For both of these major intersections, Issaquah states that delay is much larger than Sammamish. Peak hour commuters who use these intersections will almost certainly say that the Issaquah score is more accurate than the Sammamish.
| ELSP heading S at roundabout AM |
| Isssaquah Fall City Issaquah Pine Lake AM |
State Route 202 commuters
There are three major SR 202 intersections for Sammamish commuters: East Lake Sammamish Parkway, Sahalee Way NE, and 244th Ave NE. Both Sammamish and the Washington State Department of Transportation calculate delay for these intersections. The results do not agree.
AM (7-8) peak hour LOS
| Intersection | Sammamish | WSDOT |
|---|---|---|
| East Lake Sammamish Parkway and SR 202 | D | F |
| Sahalee Way NE and SR 202 | B | D |
| 244th Ave NE and SR 202 | B | C |
PM (5-6) peak hour LOS
| Intersection | Sammamish | WSDOT |
|---|---|---|
| East Lake Sammamish Parkway and SR 202 | E | E |
| Sahalee Way NE and SR 202 | C | F |
| 244th Ave NE and SR 202 | B | C |
The LOS scores produced by Sammamish are more optimistic in almost every case than those produced by WSDOT. On average, the Sammamish scores correspond to delays that are less than half those from WSDOT.
Who is right? Most commuters would likely say that their experience is at least as bad as WSDOT reports.
| Route 202 PM |
| ELSP Route 202 AM | ELSP Route 202 PM |
| Sahalee Route 202 AM | Sahalee Route 202 PM |
Near Town Center
There is also some evidence that the delays currently computed for intersections inside the city are less than observed. They will be discussed in another paper.
| 228th AM | 228th PM |
Conclusion
The significant discrepancies between the Sammamish intersection delay values and those of other jurisdictions is cause for concern. It is striking that the delays computed by Sammamish are almost always less than the others. The Sammamish results also appear to be more optimistic than the conditions that our commuters actually encounter.
These concerns about the calculations for current (2023) conditions cast doubt on the accuracy of predictions for 20 years in the future with a 20% or more larger Sammamish population. This is especially an issue because the impact on intersections of adding more cars is nonlinear. A 20% increment in volume for an A rated intersection could cause not much change in LOS. A 20% increment for a D or E rated intersection would likely be more severe.
The delay computation process should be audited before committing the city to a major growth path.
Signalized Intersection LOS Descriptions from the Highway Capacity Manual
| LOS | Delay (sec) |
|---|---|
| A | 0 to 10 |
| B | 10 to 20 |
| C | 20 to 35 |
| D | 35 to 55 |
| E | 55 to 80 |
| F | 80 or more |
LOS A Free flow with very low delay. Most vehicles arrive during the green phase and pass through without stopping.
LOS B Stable flow with minimal delay. Good progression and/or short signal cycle lengths. More vehicles stop than in LOS A, but most still pass through without significant wait.
LOS C Stable flow with acceptable delay. Progression is fair, and cycle lengths may be moderate. An occasional approach phase may be fully utilized.
LOS D Approaching unstable flow. Significant delay. Drivers often feel restricted, and may have to wait through more than one red light. This is often the acceptable standard for urban peak-hour traffic.
LOS E Unstable flow. High delay. The intersection operates at or near its capacity. Long queues and frequent cycle failures are common.
LOS F Forced flow or breakdown. Excessive delay and gridlock are likely. Demand exceeds capacity, and queues may block upstream intersections. This is generally considered unacceptable.
Sources
City of Sammamish Transportation Master Plan, 2024
Sammamish Town Center Plan & Code Amendment Draft SEIS, 2025
City of Sammamish Comprehensive Plan, 2015
City of Issaquah Mobility Action Plan, 2025
SR 202 Corridor Study, WSDOT 2019
I would trust WDOT, Sammamish has a conflict of interest.
ReplyDelete