When a signal corridor experiences recurring congestion, unreliable travel times, or frequent complaints, it can be difficult to determine where to begin. Multiple intersections may appear problematic, but limited staff time often makes it impractical to investigate every signal in detail.
Corridor triage provides a structured, data-driven approach for identifying the intersections contributing most to poor corridor performance. By combining corridor travel time analysis with intersection-level metrics such as delay, excessive delay, and arrivals on green, traffic engineers can quickly prioritize problem locations and focus improvement efforts where they are most likely to make an impact.
This workflow helps agencies move beyond guesswork and develop a repeatable process for improving corridor travel times and reliability.
When to Use This Approach
This workflow is particularly useful when:
- You’re receiving multiple complaints along an arterial corridor
- Travel times are unreliable, but cameras don’t point to a single issue
- You need to determine which intersections are causing the biggest problems
- Staffing limitations prevent detailed reviews of every signal in detail
- You want a repeatable, defensible workflow for corridor reviews
What Data Do You Need?
One advantage of this approach is that it requires very little upfront data collection.
Required inputs:
- A defined set of adjacent signals
- A consistent analysis window (e.g., specific dates, weekday AM peak or PM peak)
- Access to movement-level delay, arrivals on green, and excessive delay
Not required:
- Signal timing plans
- Detector data
- New field counts
With the right performance metrics already available, engineers can begin evaluating corridor performance immediately.
Step-by-step: A Corridor Triage Workflow That Works
Step 1: Define the Corridor the Way Drivers Use It
Define corridor start and end points using observed turn counts data, not just what’s convenient on a map.
Corridor analytics work best when:
- Most trips complete the full start‑to‑end segment
- Corridors are not so long that few vehicles traverse the entire length
- Overlapping corridors are used to reflect real travel patterns
This ensures travel time, delay, and reliability metrics are meaningful rather than diluted.
Step 2: Check Corridor Level Performance First
Before diving into individual signals, review:
- Corridor travel time and reliability
- Time‑of‑day patterns (AM vs PM vs off-peak)
- Repeatability across multiple day of the week
If poor performance appears consistently across multiple days, it’s time to investigate the intersections contributing most to the problem.
Step 3: Rank Intersections by Delay and Excessive Delay
Once it’s confirmed that a corridor issue exists, shift the focus to individual intersections.
Within the corridor:
- Rank intersections by average control delay
- Separately rank by excessive delay (vehicles with more than 3 minutes of control delay)
- Look for intersections that appear near the top of both lists
These are often the locations doing the most damage to corridor performance.
Step 4: Use Arrivals on Green to Validate Progression
Arrivals on green help answer a critical question:
Are vehicles arriving during green phases—or stopping because coordination is failing?
Low arrivals on green for major coordinated movements often indicate:
- Offset issues
- Mismatched progression speeds
- Downstream queues spilling back and breaking platoons
Arrivals on green provides valuable context before making timing changes and helps distinguish progression issues from capacity constraints.
Step 5: Narrow the Focus to the Top 2-3 Intersections
Most corridors don’t require network-wide fixes.
A practical rule:
- Focus first on the two or three intersections showing the highest delay and the most consistent excessive delays during peak periods
- Deprioritize isolated one‑day spikes caused by incidents or construction
Concentrating efforts on a small number of consistently problematic locations keeps reviews manageable and increases the likelihood of measurable improvements.
Step 6: Decide What to Fix (Before Going to the Field)
Based on patterns you see:
- High excessive delays on one movement → likely green time, demand or storage issue
- Low arrivals on green corridor-wide → likely coordination or offset issue
- High delay across all movements → possible cycle length or phasing problem
This prioritization gives field investigations purpose instead of relying on guesswork.
Step 7: Measure Results After Changes Are Made
After retiming or adjustments:
- Compare before/after using the same time‑of‑day windows
- Confirm reductions in delay and excessive delay persist over multiple days
- Check that improvements didn’t simply shift problems elsewhere
This closes the loop and creates defensible documentation.
Step 8: Refine Based On Real-World Measurements
Small adjustments to split times, offsets, or side-street green times can have a significant impact on operations. These opportunities are often overlooked because simulations cannot fully capture local driving behavior, weather conditions, school schedules, or other environmental factors.
What Does Success Look Like?
Performance targets vary by corridor, but successful corridor optimization often includes:
- Fewer movements experiencing frequent excessive delay
- Higher arrivals on green for coordinated movements
- Reduced corridor travel time variability
- Improvements that remain consistent across weekdays
Absolute targets vary by context; trends and repeatability matter more than any single number.
Common Pitfalls to Avoid:
Even with strong analytics, several issues can lead to misleading conclusions. Watch for:
- Overly long corridors with few complete trips
- Construction periods contaminating results
- Low volume movements requiring longer analysis windows
- Assuming metrics replace engineering judgment
Analytics help prioritize effort. Engineering judgment determines the right solution.
What Should Be Included in the Final Deliverable?
Most agencies summarize corridor triage findings with:
- A ranked list of problem intersections
- Before/after snapshots for priority locations
- Notes explaining why fixes were prioritized
This makes coordination easier with consultants, management, and leadership.
Turning Data Into Action
Corridor triage is ultimately about focusing limited resources where they can have the greatest impact.
By combining corridor travel time analysis with intersection-level delay, excessive delay, and arrivals on green, engineers can quickly identify the locations most responsible for poor corridor performance, develop targeted improvement strategies, and verify results after implementation.
The result is a faster, more defensible workflow that helps agencies move from identifying problems to solving them.
Want to learn more about the U.S. Signals Scorecard? Join us for the webinar on July 29th.