Find Your Ambulance Number

Some patients don’t have the luxury of time



Gary Ludwig, MS, EMT-P | From the July 2010 Issue | Thursday, July 1, 2010

The other day, I took a flight to Washington, D.C., to attend some meetings at the National Fire Academy. I stood in line to get my ticket at the kiosk. I stood in another line to go through security. Then, I stood in a line to get on the plane. It made me reflect on the fact that I’d also stood in a lot of other lines in the past week.

The Problem
The reason I stood in so many lines was because there were fewer workers than people standing in the line. When I waited at the bank, there were three tellers working and five customers waiting. If the bank had eight tellers, nobody would’ve waited. However, if the bank had only one teller and eight customers, the other seven waiting customers would be unhappy—especially the last one, which always seems to be me.

The reason the bank has only three tellers is because it doesn’t always need more employees. If the bank were consistently staffed with eight tellers, they’d often be idle, and that wouldn’t be cost effective. Consequently, the bank managers might be criticized for this inefficiency.

Those managing an EMS agency face the same dilemma. How many ambulances are needed to run an EMS system? If there are too many ambulances not busy throughout a tour of duty, the EMS manager can be criticized for having an over-inflated, ineffective budget. But if you have too few ambulances, patients (i.e., customers) wait to receive service.

The key difference between EMS and a bank is that when our customers wait too long, it may result in deaths, lawsuits and unfavorable media coverage. Can you imagine a 9-1-1 system with calls in a queue as the dispatchers wait for ambulances to come in service? On the other hand, no EMS system can afford to park ambulances on every corner.

So what’s the answer?
This is one of the most frequent questions I hear: How many ambulances should a community have? My answer is … it depends.
I’m sure some of my fellow EMS managers will take issue with my opinion, but I don’t think you can base the number of ambulances for a community solely on population. I’ve heard rules of thumb in the past, such as one ambulance for every 30,000 people. I disagree.

In my experience, urban EMS systems need a higher ratio of ambulances per population than a suburban or rural community. Additionally, instead of trying to estimate the number of ambulances based on population, I believe the number of ambulances deployed should be based on 9-1-1 call volume and response time. For career fire departments, EMS response-time guidelines can be found in the National Fire Protection Association 1710 standard that dictates an eight-minute response time for ALS 90% of the time.

The Solution
Some EMS managers apply the Queuing Theory for ambulance deployment and how to best serve their customers. The theory is the mathematical study of waiting lines and the consequences of those lines. If the Queuing Theory is applied to EMS, it would be used to examine several processes, including the time delay between when a call is received and when it’s dispatched. This would be especially true if the dispatcher must hold the call in a queue until an ambulance becomes available.

Upon ambulance arrival, you can also measure the time it takes before providers make contact with the patient. This is an important factor, especially in large, urban areas where high-rises can delay contact with the patient another five minutes or more.

The primary purpose of the Queuing Theory is to decide how many ambulances are needed during a given hour of the day. Data collection is necessary to measure the average number of calls your EMS system receives in an hour.

To determine this data, EMS managers typically employ unit hour utilization (UHU). Sophisticated software programs can now take computer-aided dispatching data and calculate the average number of calls in the day and thus, the UHU for your EMS system.

Furthermore, data mining can determine where delays occur in the system by crew, hospital, geographic location, etc. Some EMS systems measure UHU differently. Agencies may measure only calls that include patient transport, while other systems also include cancelled calls, patient refusals or standbys. You could factor in vehicle breakdowns and the time a unit is out of service for training or administrative issues.

Regardless of which factors you decide are most relevant to your study, the Queuing Theory is a beneficial tool in determining the proper number of ambulances needed in an EMS system. JEMS

This article originally appeared in July 2010 JEMS as “The Waiting Game: Some patients don’t have the luxury of time”


Connect: Have a thought or feedback about this? Add your comment now
Related Topics: Administration and Leadership, Operations and Protcols, KEYWORDS: “Queuing theory”; “ambulances in service”; “ambulances”, Jems Leadership Sector

Author Thumb

Gary Ludwig, MS, EMT-P

is a deputy fire chief with the Memphis (TN) Fire Department. He has over 35 years of fire, EMS, and rescue experience. He is also the immediate past Chair of the EMS Section for the IAFC. He can be reached at


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