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Public Health Emergency Preparedness

This resource was part of AHRQ's Public Health Emergency Preparedness (PHEP) program, which was discontinued on June 30, 2011, in a realignment of Federal efforts.

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Appendix 3: Los Angeles Pilot Test

In this appendix, the modeling assumptions and output from the Los Angeles pilot test are described. The scenario involved a major (7.1) earthquake that resulted in the need to evacuate three hospitals.

Modeling Assumptions

The modeling assumptions for the baseline evacuation scenario are described below.

Vehicles used in the evacuation

  • Four types of vehicles are used to transport patients—buses, wheel chair vans, BLS ambulances, and ALS ambulances. The capacities of the vehicles are:
Vehicle Type Capacity (Patients)
Bus 34 ambulatory patients
Wheel chair van 3 wheel chair patients
BLS ambulance 2
ALS ambulance 1
  • The number of vehicles, by type, that are available for the evacuation:
Vehicle Type Number available Assumptions
Bus 125 5% of the fleet (2500)
Wheel chair van 15 5% of the fleet (300)
BLS ambulance 38 5% of the fleet (750)
ALS ambulance 12 5% of the fleet (240)

Patients to be evacuated

  • 3 hospitals are evacuated. The three are: Kaiser/Los Angeles, Children's Hospital/Los Angeles, and Hollywood Presbyterian. The three are all located within 4 blocks of each other.
  • Number of patients to evacuate. Each of the three hospitals provided census figures by patient type:
Hospital Patient Type
Average Daily Census # in ICU/CCU # on Ventilators # on Monitors
Kaiser/Los Angeles 295 54 25 180
Children's/Los Angeles 254 65 47 131
Hollywood Presbyterian 279 25 9 25
Total 828 144 81 336

We also assumed that there would be patients in the Emergency Department (ED) that would need to be evacuated. Typically, the number of patients in the ED is between 5 and 10 percent of the average daily census; we assumed 7.5 percent. Including the ED patients increased the total number of patients to be evacuated to 890:

Hospital Average Daily Census Add in Estimated # ED Patients Total Evacuated
Kaiser/Los Angeles 295 22 317
Children's/Los Angeles 254 19 273
Hollywood Presbyterian 279 21 300
Total 828 62 890
  • No patients will self-evacuate. All evacuated patients will be transported to another facility on one of the vehicles described above.
  • Evacuated patients are divided into 4 types. Each patient type is assumed to be evacuated on a specific type of vehicle:
Patient Type Vehicle Used for Patient Transport
Ambulatory patients Bus
Wheel chair bound patients Ambulette
Bedridden patients requiring constant medical attention at the BLS level during transport BLS ambulance
Bedridden patients who need constant medical attention at the ALS level during transport ALS ambulance
  • The percentage of patients requiring each vehicle type for transport varies by evacuating hospitals. To determine the number of patients that require a bus, wheel chair van, BLS, and ALS ambulance, we assumed (1) all patients on monitors will need ALS transport; (2) admitted patients not on monitors are divided evenly between BLS, wheelchair van, and bus transport; and (3) ED patients are divided evenly between ALS, BLS, wheelchair van, and bus. With these assumptions, roughly 40 percent of patients required an ALS ambulance and 20 percent each required a BLS ambulance, wheel chair van, and bus:
Hospital # Needing ALS # Can Use BLS # Can Use Wheelchair Van # Can Use Bus Total
Kaiser/Los Angeles 186 44 44 44 317
Children's/Los Angeles 136 46 46 46 273
Hollywood Presbyterian 30 90 90 90 300
Total 352 180 180 180 890
% of Total 39% 20% 20% 20% 100%

Facilities receiving evacuated patients

  • Any hospital in Los Angeles County (other than the three evacuating hospitals) will accept evacuating patients.
  • Receiving facilities are at 100% capacity.
  • Receiving facilities have 15% surge capacity. That is, the number of patients a facility can accept is equal to 15% of their capacity.
  • The model does not consider the capacity of different units within a receiving facility. An evacuated patient can be placed in any bed in the receiving facility. In other words, we assume acuity affects only the transport vehicle requirement, and not the bed requirements in the receiving facility (e.g., an ICU patient in an evacuating hospital can be relocated to a floor room, with additional equipment brought in, at the receiving hospital).

Staff available to carry out the evacuation

  • The model does not consider staff availability. We assume that:
    • Sufficient staff are available to move patients from their room to the lobby of the facility.
    • Sufficient facility staff are available to accompany patients in the vehicle to the receiving facility, if required (e.g., high-risk patients or minors).
    • Sufficient drivers, EMTs, and paramedics are available to staff the vehicles 24/7 during the entire evacuation period.

Steps in the evacuation process

  • Facility and/or vehicle staff help move patients from their beds onto the vehicles. The assumed loading time for vehicles per patient is:
Vehicle Type Loading Time (Minutes)
Bus 1
Wheel chair van 5
BLS ambulance 10
ALS ambulance 20
  • Evacuated patients will be transported to the nearest available facility that has capacity, with two exceptions. The exceptions are: Kaiser patients are only transported to other Kaiser hospitals, and the ICU patients at Children's Hospital are only transported to hospitals that have pediatric ICU units. Other than these two exceptions, the model ignores "preferred" receiving facility lists for each evacuating facility (e.g., pairs of facilities with MOUs regarding evacuation).
  • The model sends ALS patients to the nearest facilities. Given that assignment, the model sends BLS patients to the nearest facilities that still have available capacity. Van patients have the next highest priority, and bus patients have the lowest priority.
  • All three hospitals are evacuated simultaneously, depending on vehicle availability.
  • Travel times between facilities are based on straight-line distance and population density. In the model, all vehicles are assumed to encounter the same traffic. To estimate the travel times between all pairs of facilities, we used known travel times for the inter-location travel between all pairings of 10 selected locations to create a model of travel time based solely on geographic distance and characteristics of the city. We assumed that travel times during the evacuation were twice the time during normal conditions.
  • The time required to unload patients from a vehicle is the same as the loading time. The model assumes no vehicular congestion at the facilities that would delay patient unloading.
  • Staff at the receiving facility and/or vehicle staff move patients from the vehicle to the patient's new room.

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Modeling Results

Results with baseline assumptions

Using the baseline assumptions described above, the estimated total evacuation time is 105 hours.

Total evacuation time depends entirely on transport of ALS patients. The model predicts that the total time to evacuate ALS patients far exceeds the time required to evacuate BLS, van, or bus patients. Therefore, the total evacuation time is determined by the total time to evacuate ALS patients from facilities (primarily hospitals). Although the majority of patients can be moved in wheelchair vans or buses, the small number of ALS ambulances (12) and the fact that they can only move one patient at a time means that those patients (374 of them) take the longest time to evacuate.

The model prioritizes ALS ambulances to go the shortest distance (to limit travel time for those patients), but they make on average 32 round trips during the evacuation (see the table below). BLS ambulances taking two patients each and with a greater fleet of vehicles make only 2 trips each. Wheelchair vans can take more people but there is a much greater pool of patients to transport. These vans each make six trips to evacuate. Buses are plentiful and each only needs to make one trip in order to evacuate all ambulatory patients. The number of round trips required during the evacuation for each vehicle is summarized below:

Vehicle Type Number of Round-Trips Required per Vehicle
Bus 1-2
Wheel chair van 3-4
BLS ambulance 2
ALS ambulance 32

All patients would be transported to hospitals within Los Angeles County.

ALS patient evacuation time under alternative assumptions

The figures below, which were presented at the Los Angeles table top exercise, show how different model inputs affect total evacuation time. In each exhibit we have assumed all of the baseline assumptions stated above except for the parameter that is shown on the X axis.

Traffic Congestion

A key variable in determining total evacuation time is the traffic congestion. The graph below shows the evacuation time, in days, if the travel time is multiplied by different factors above normal travel times. The baseline evacuation time assumed a travel time multiplier of two.

Graph depicts Effect of Traffic Congestion on Evacuation Time.  For details, go to [D] Text Description.

[D] Select for Text Description

Number of Available ALS Ambulances

The effect of the number of the available ALS ambulances on total evacuation time is shown in the figure below. Unlike traffic congestion, the effect is non-linear—as the number of ALS decreases, the evacuation time increases be a disproportionate amount. The policy implication of this exhibit is the importance of increasing the number of available ALS ambulances from 12 (the baseline assumption) to at least 20 or 25. 

Graph depicts Effect of ALS Availability on Evacuation Time.  For details, go to [D] Text Description.

[D] Select for Text Description

Loading Time

The linear effect of changes in patient loading time is shown below.

Graph depicts Effect of Patient Loading Time on Evacuation Time.  For details, go to [D] Text Description

[D] Select for Text Description

Standard of Care

The exhibit below shows the effects on evacuation time from transporting some ALS patients on BLS ambulances. "Standard" is the baseline assumption (40 percent of patients require ALS ambulances and 20 percent require BLS ambulances). The next category shows the time if half of the ALS patients were transported via BLS. The last shows the time if the proportions were reversed, i.e. if 40% of patients required BLS and only 20% ALS transport. This takes advantage of the greater number of BLS ambulances and the greater capacity of BLS ambulances. The exhibit illustrates the dramatic effect on evaluation time of reclassifying patients in this manner. (The model does not estimate the "costs" resulting from the reduced standard of care.

Graph depicts Effect of Standard of Care on Evacuation Time.  For details, go to [D] Text Description.

[D] Select for Text Description

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AHRQ Publication No. 080102-1-EF
Current as of August 2008

The information on this page is archived and provided for reference purposes only.


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