Understanding the Numbers

Two numbers that determine whether development gets approved

Volume-to-Capacity Ratio (V/C)

V/C is the simplest concept in traffic engineering: how much traffic is on the road compared to how much it can handle. A V/C of 0.50 means the road is half full. A V/C of 1.00 means it’s at capacity. Anything over 1.00 means more vehicles are trying to use it than it can physically process.

Clark County’s threshold is V/C = 0.90. Anything above that is considered failing. That 10% buffer exists because traffic isn’t perfectly smooth—small disruptions cascade quickly once a road is near capacity.

The V/C ratio is what the County uses to evaluate road segments—stretches of road between intersections. When a traffic study says a segment is “failing,” it means the V/C exceeds 0.90.

Level of Service (LOS)

LOS is a letter grade from A to F that measures how long you wait at an intersection. It’s based on seconds of delay per vehicle. Think of it like a report card for your commute:

How V/C and LOS relate—and where they diverge

V/C measures how full the road is. LOS measures how long you wait. They usually track together—a full road means long waits. But at roundabouts, they can tell completely different stories. That’s where the County’s system breaks down.

Why an intersection graded “A” can actually be failing

Roundabouts are designed for high throughput with low delay. Traffic flows continuously instead of stopping at a red light. That means even a very busy roundabout can report low average delay—and therefore a good LOS letter grade—while individual approaches are completely overwhelmed.

The reason is simple: roundabout LOS is a volume-weighted average across all approaches. If 1,700 vehicles on NE 179th sail through at LOS A, and 700 vehicles trying to enter from the I-5 ramp are stuck at LOS F with 88-second delays, the average comes out to LOS C. The report says “LOS C” and staff use that to conclude the intersection is operating acceptably.

Here’s what that actually looks like, using real data from the Three Creeks East traffic study:

The 85% design threshold

Engineering practice holds that roundabouts should be designed to operate at or below 85% of their theoretical capacity (V/C 0.85). Above that, performance degrades rapidly—small increases in volume produce disproportionate increases in delay, and the roundabout becomes unreliable. Several approaches in the corridor studies are already well above this threshold.

The same intersection, two different answers

At the I-5 NB Ramps roundabout, the Towhee Creek study found V/C 0.822 and LOS A (passing). The NE 174th Street study—prepared by the same consulting firm, using the same horizon year—found V/C 1.138 and LOS F (failing). The difference: roughly 87 vehicles per hour in background traffic assumptions.

Part of the reason for that difference is that five traffic studies were prepared during the same time period and none of them count each other’s traffic. The code requires in-process developments to be included in background assumptions—but it isn’t happening. So the 87-vehicle gap isn’t just an unexplained discrepancy—it’s likely a larger failure than either study shows, because both are missing trips from the other concurrent applications. The system has no mechanism to reconcile these contradictory results, and both studies were used to support development approvals.

These numbers are a floor, not a ceiling

Every number in the tables above understates the actual problem. Here’s why.

Five studies don’t count each other

Five traffic studies were prepared for corridor projects during the same time period. None of them include the traffic from the other four in their background assumptions. The County’s code requires in-process developments to be included—but it isn’t happening. Hundreds of peak-hour trips loading the same roads simply don’t appear in each other’s analysis. The same engineer prepared multiple studies using the same horizon year, and none included the others as “in-process” developments. If all concurrent applications were loaded simultaneously, V/C ratios would climb higher and segments currently shown as passing could cross the 0.90 threshold.

Distance caps exclude the worst bottlenecks

A project generating fewer than 50 peak-hour trips only has to analyze roads within one mile. Most corridor developments fall into this category. The I-5 interchange—the corridor’s biggest constraint—may fall outside that radius entirely. The Salmon Creek commercial district where residents shop, get gas, and see the doctor is also outside the study area for most projects. The GMA contains no geographic limitation on concurrency evaluation. The distance caps are a County-level creation.

Ghost capacity inflates the denominator

When a traffic study calculates V/C for a road segment, the “capacity” number used is based on the road as it’s planned to exist—not as it exists today. NE 179th Street is physically a two-lane rural road (capacity ~600 vehicles per hour), but the Arterial Atlas designates it as a future multi-lane arterial (capacity 1,800 vehicles per hour). Using 1,800 as the denominator makes the V/C ratio look three times better than the actual road conditions warrant. One consultant—Kittelson, preparing the Kozy Manor study—correctly used the reduced capacity. The others used the inflated number.

Reserved trips aren’t on the road yet

WSU Vancouver and commercial developments hold reserved trips under concurrency agreements and submitted plans. These trips are committed capacity—they count against the system’s limits—but they’re not generating traffic yet. Current traffic counts don’t reflect the full committed load. When buildout occurs, the actual conditions will be worse than what current counts suggest.