Economist Jack Stout developed Unit-Hour-Utilization (UHU) in the late 1970s as part of the Public Utility Model for high-performance EMS: “A ‘unit hour’ is simply a fully equipped and manned ambulance on the street for one hour” (Stout, 1983). In the pre- computer aided dispatch, digital maps, and automatic vehicle locator era, a back of the envelope estimate was that each ambulance response with transport would take one hour to complete. Transports are the only source of revenue.
Dan Munsey’s Executive Fire Officer applied research project on firefighter workload efficiency included an extensive literature review of how UHU is determined and which thresholds are used. The literature showed wide variation in definitions and in how UHU is calculated and applied (2018). Stout’s framing was managerial rather than mathematical. He defined the unit hour and argued that dispatchers must “utilize the available ‘unit hours’ in the best way…to squeeze the highest response time performance possible.” He did not publish a UHU percentage formula in the 1983 article; consulting firms and industry bodies later codified UHU ratio conventions and thresholds.
Fuzzy UHU Conventions, Thresholds, and Two Examples
No national consensus standard or benchmark exists for unit hour utilization. The measurement started with the Public Utility Model, which featured high-volume, dynamically deployed paramedic ambulances in cities. Recent examples use unit time-on-task to expand the UHU analysis to include emergency responses without transport and the cost of readiness. Cost of readiness includes the on-duty training, call documentation, post-call recovery, and return-from-hospital time.
Baltimore
FACETS performed an operational and management assessment of the Baltimore City Fire Department. At that time, Baltimore had the second busiest fire-based ambulances in the United States.1
The workload among BCFD unit types is highest for the ALS and BLS ambulances (ranging from 39.27% [9.4 hrs] to 71.07% [17.1 hrs]). These numbers are exceptionally high, especially for employees working a 24-hour shift.
Even the least busy ambulance exceeds the workload percentage recognized by many agencies for 24-hour shift durations (30% or 7.2 hrs UHU 0.3) by almost 10 percent.
This level of workload can result in substantial employee wellness issues, does not provide on-duty time for essential training, and also leads to extended response times at the current level of resource deployment. Moreover, fatigue can negatively affect patient care and decision-making at incidents. (FACETS, 2021, pp. 48-49)
Photo from amott919 (2025, October 1)
FACETS used workload data from 2018-2019. The busiest EMS unit was on task – that is, responding to, at scene, transporting to the hospital, and waiting to clear the hospital – for 17.1 hours in a 24-hour workday. That is a UHU of 0.71.
FACETS, Citygate, Fitch, and other consulting groups are using a UHU of 0.3 as the upper workload limit for 24 hour 9-1-1 units. That is 7.2 hours time-on-task in a 24 hour workday,
Los Angeles City
Citygate Associates 2023 Standards of Cover analysis of the Los Angeles Fire Department observed:
Citygate recommends the LAFD add at least 14 additional rescue ambulances (both ALS And BLS to relieve the busiest types), … Further, there are currently at least 25 rescue ambulances on 24-hour shift staffing that are overworked for excessively long periods of a 24-hour day. Citygate does not believe that critical patient care, much less safe firefighting, is always possible when a crew has gone from call to call for 12 or more hours. (Citygate Associates, 2023, p. 7)
In a 2020 hour-by-hour analysis, LAFD rescue ambulances peaked at 0.62 UHU and experienced up to 15 consecutive hours above 0.40. Citygate recommended changing staffing for the 25 busiest rescue ambulances from 24-hour to 12-hour shifts.
Screenshot from 90210firebuff (2025, January 24)
Here is a summary of their recommendations:
Battalions 1 and 11
- Station 3: 12 hour shifts for both rescue ambulances
- Station 4: Add fourth rescue ambulance
- Station 6: 12 hour shifts for both rescue ambulances
- Station 10: 12 hour shifts for both rescue ambulances
- Station 11: Add third rescue ambulance
- Station 13: 12 hour shifts Rescue Ambulance 13
Battalion 13
- Station 33: Add third ambulance
- Station 46: Add third ambulance
- Station 57: Add fourth ambulance and go to 12 hour shifts for the three existing rescue ambulances
- Station 64: Add fourth ambulance and go to 12 hour shifts for the three existing rescue ambulances
- Station 66: Add fourth rescue ambulance
Battalions 5 and 18
- Station 27: Add third ambulance and go to 12 hour shifts for the two existing rescue ambulances
- Station 58: Add fourth ambulance and go to 12 hour shifts for the three existing rescue ambulances
- Station 61: Add third ambulance and go to 12 hour shifts for the two existing rescue ambulances
Northern Areas
- Station 39: 12 hour shifts Rescue Ambulance 39
- Station 60: 12 hour shifts for both rescue ambulances
- Station 89: Add third ambulance and go to 12 hour shifts for the two existing rescue ambulances
Northern Area: Battalion 12
- Station 7: Add second rescue ambulance
- Station 98: 12 hour shifts for both rescue ambulances
UHU was invented to optimize a dynamically deployed, transport-focused, revenue-sensitive ambulance service and was later imported into fire-based EMS, where units are fixed-post, cross-staffed, and expected to carry a substantial non-emergency workload.
Better Defining Real Work in a UHU Calculation
Wendy Korotkin and Alex Stephenson point out that the UHU metric used to calculate ambulance economics was not designed to measure workload, but no better option exists. Their study identified four operational factors that contribute to cumulative fatigue:
- Night wake-up
- Burst
- Volume
- Call severity
Night Wake-up
Calls from 10pm to 6am trigger a coefficient that grows with each successive interruption: 1.25 for the first wake-up, 1.50 for the second, 1.75 for the third, plus 0.25 for every additional disturbance. Sleep debt is not a step function. Each interruption costs more than the last, and recovery within a single shift does not occur.
Burst
When a unit leaves one call and is dispatched to another within 15 minutes, the second call is flagged as a “burst.” Daytime bursts carry a coefficient of 1.25, and overnight bursts, 1.50. The factor accounts for rehydration, decontamination, report writing, and rest time lost due to back-to-back assignments.
Volume (per 24-hour shift):
Volume is binned by the cumulative call count: no adjustment up to five calls, 1.05 above five, 1.25 above ten, 1.50 above 15, and 1.75 above 20 calls. Volume also serves as a proxy for the volume of administrative work (report writing, equipment checks, restocking) that scales with call count and is invisible to UHU.
Severity
Two incident types were used: structure fires with more than 90 minutes of on-scene time and cardiac arrests with documented compressions. The severity of structure fires increases with time on scene, from 1.50 at 90 minutes to 2.50 after 3 hours. For cardiac arrests, severity is set at 1.50 for adults and 3.00 for children. These values account for both bodily stress and the mental impact of resuscitation, which is significant with pediatric patients. When none of these four conditions are met, the coefficient defaults to 1.0, making the weighted measure standard. Differences between the two occur only when the operating environment includes conditions UHU was never built to register.
The compounding effect of the work becomes clearer through an example. Consider Engine 23, dispatched to a structure fire at 2:30 am for an incident lasting 140 minutes. Three factors influence the timing: it’s the twelfth call of the shift, the crew’s second wake-up, and eight minutes after the last call.
Standard UHU says the crew spent 140 minutes on the call. The weighted measure tells a different story: the call was equivalent to 689 minutes. In this example, the multiplier is 4.92, the product of severity (1.75 for a structure fire above the two-hour threshold), burst (1.50 for an overnight burst), volume (1.25 for the 10-call threshold), and night wake-up (1.50 for the second wake-up). No single factor drives the gap. The compounding does.
What Does This Mean For You?
The weighted measure does not replace UHU. Ambulance economics still needs a denominator built from staffed unit hours, and the budget office still needs a productivity ratio it can defend. The Korotkin and Stephenson framework asks a different question. It asks what the shift cost the crew.
Engine 23’s 140 minutes on scene will show up on the run sheet, in the CAD extract, and in next quarter’s utilization report as 140 minutes. The crew that cleared at 4:50 am, restocked the rig, decontaminated turnout gear, finished the report, and tried to sleep before the 6 am tones will tell you something closer to 689. Both numbers are true. Only one of them predicts whether that paramedic is still riding the seat in five years.
Three Points To Explore
Run the numbers on your own houses. Pull ninety days of National Emergency Response Information System (NERIS) and electronic Patient Care (ePCR) data, apply the four coefficients, and rank your companies by weighted load rather than raw call count. Those are the crews to watch for sick-leave clustering, off-duty injury, disciplinary spikes, and voluntary transfer requests.
Identify the compounding shifts before they happen. Night wake-up and burst are knowable in near real time. A Computer Aided Dispatch (CAD) rule that flags any unit hitting a second overnight run within fifteen minutes, or a tenth call before 4 am, gives the on-duty chief a decision point: hold the unit down for the next low-acuity call, cross-staff from a slower house, or bring in an off-duty crew on overtime. You will need to add the trigger logic within the CAD system.
Build recovery into the deployment plan. After a structure fire past the two-hour mark or a worked pediatric arrest, the crew comes out of service for a defined interval. Not at the officer’s discretion, not contingent on call volume, not subject to backfill availability. Peak-demand units, jump companies, and mutual-aid agreements are the mechanisms. The weighted measure is the justification.
UHU was built to price an ambulance. It was never built to measure what the work does to the people doing it. Until something better arrives, a measure that accounts for when the call happened, what came before it, how many preceded it, and what it demanded is the closest the fire service has to an honest number.
Try this for a month. Pick one battalion, one pay period, and the four coefficients above. Run standard UHU and weighted UHU side by side. If the two numbers agree, your deployment model is absorbing the load. If they diverge, you have found a more informed way to describe the emergency response workload.
__________________________________
1Philadelphia, Los Angeles City, Baltimore, and Chicago are the four busiest fire-based ambulance services in 2026.
References
Citygate Associates. (2023) Los Angeles Fire Department Standards of Cover Analysis. Volume 1 of 3: Technical Report. Citygate Associates LLC. https://ens.lacity.org/lafd/lafdreportarchv/lafdlafdreport1864172030_06012023.pdf
FACETS (2021) Operational & Management Assessment: Baltimore City Fire Department – Final Report. FACETS Consulting LLC.
Korotkin, W., & Stephenson, A. (2026). Not All Time Is Created Equal: In Firefighting, Time-Based Metrics Can Miss the Real Work. ORMS Today. https://doi.org/10.1287/orms.2026.03.03. https://pubsonline.informs.org/do/10.1287/orms.2026.03.03/full/
Munsey, D. (2018). Determining Workload Efficiency for San Bernardino County Fire Protection District Suppression Crews. [Executive Fire Officer Program applied research project, National Fire Academy]. U.S. Fire Administration. https://apps.usfa.fema.gov/pdf/efop/efo247258.pdf
Patterson, P., Martin, S., MacAllister, S., Weaver, M., & Patterson, C. (2025). Variations in Sleep, Fatigue, and Difficulty with Concentration Among Emergency Medical Services Clinicians During Shifts of Different Durations. International Journal of Environmental Research and Public Health, 22. https://doi.org/10.3390/ijerph22040573.
Stout, J. (1983, May) System Status Management: The Strategy of Ambulance Placement. Journal of Emergency Medical Services. https://emsmuseum.org/wp-content/uploads/2021/05/System-Status-Mangement-The-Strategy-of-Ambulance-Placement-22-32-1983-05-JEMS.pdf
Feature picture from Emergency Monologues. https://x.com/emergemonos