Vertical Transportation Design and Traffic Calculations – Part Six

Today we will start explaining the lift traffic design calculations. In article “Vertical Transportation Design and Traffic Calculations – Part Two”, we listed the (4) methods of Traffic Design Calculations, which were:
First: Calculation methods, which includes:
1- The Formula-Based Method (Classical Method Method),
2- The Monte Carlo Simulation Method.

Second: Simulation methods, which includes:
3- Discrete Event Simulation Method,
4- Time Slice Simulation Method.

Also, we indicted that in this course, we will explain only the first method which is The Formula-Based Method (Classical Method) applied to commercial office buildings.

Traffic Design Calculations


In order to get a better understanding of the traffic design calculations by The Formula-Based Method (Classical Method). We need to know the following:
  1. The Daily Routine Work of an Elevator,
  2. The Round Trip Cycle of an Elevator.

1- The Daily Routine Work of an Elevator

Fig.1: Passenger demand rate for an office building

The daily routine work of an elevator can be called simply the daily traffic patterns of an elevator. Fig.1 illustrates a possible traffic pattern and shows the number of up landing calls and down landing calls registered during the working day in a fixed time company where employers must start and leave the work in predetermined time, these daily traffic patterns can be divided to:

1- Morning up-peak:
At the start of the day there is a larger than average number of up-hall calls. This is due to the building’s occupants arriving to start work.

2- Evening down-peak:
Late in the day there is a larger than average number of down-hall calls. These are due to the building’s population leaving the building at the end of the working day.

3- Midday traffic:
In the middle of the day there are two separate sets of uppeaks and two down peaks. This represents a situation where the occupants of the building take two distinct lunch periods (ie: 12.00 to 13.00 and 13.00 to 14.00). This pattern is sometimes called two-way traffic.

4- Random (balanced) inter-floor traffic:
During the rest of the day the numbers of up-hall and down-hall calls are similar in size and over a period are equal.

The above traffic patterns will not be observed in a flextime company where employers can start and leave the work as per their choice.

From above explanation of the daily routine work of an elevator in a commercial office building, we can list the different traffic conditions as follows:

  1. Uppeak traffic,
  2. Down peak traffic,
  3. Two Way and Midday (Lunch Time) Traffic,
  4. Random Inter-floor Traffic.

And as we indicated above, we will be interested only in the Uppeak traffic condition used with the Formula-Based Method (Classical Method Method).

1.1 Uppeak Traffic

An uppeak traffic condition exists when the traffic flow is in an upward direction, with all, or the majority of, passengers entering the lift system at the main terminal of the building.

Fig.2: the up peak traffic profile

Fig.2 reveals the detail that the up peak traffic profile is slow to rise and quick to fall. The lift installation must be able to handle the peak if a satisfactory service is to be provided. From fig. , we have observed (3) nos. handling capacities as follows:

A- 5 minutes handling capacity
To size a lift installation to handle the number of passengers requesting service during the heaviest five minutes of the up peak traffic condition as recommended by Industry practice.

B- One hour handling capacity
To size the lift system to handle the actual peak would require too large a system, which would be very expensive and much of the equipment would be under-utilized during large periods of the working day.

C- 30-minute handling capacity
To size a lift installation to handle the peak in a 30-minute of the up peak traffic condition and this would result in a totally inadequate installation, not only for up peak traffic but also for the other traffic conditions.

  • In the morning up-peak, 5-minute segment, little or no traffic is moving interfloor or down in the building. The lifts are loading passengers at the main lobby, distributing those passengers to various upper floors and then making an express trip back to the main lobby for the next load. Therefore, studies are based upon “one-way traffic” in the up direction with no stops at the intervening floors in the down direction. It is possible to consider some down travelling passengers, but there is no consensus as to how big this flow should be. uppeak traffic flow is the best method which can be used to compare any designers results.
  • In general, if the uppeak traffic pattern is sized correctly all other traffic patterns will also be adequately served. There are exceptions to this comment. For example: in hotels at meal times; in hospitals at visiting times; in buildings with trading floors (insurance and stock markets), which open at specified times and at lunch time in all buildings.
  • From above explanation, in The Formula-Based Method (Classical Method), the lift system is recommended to be sized as per the 5 minutes up peak traffic handling capacity.
  • The Formula-Based Method will not be used for some special conditions like:

  1. The case of multiple entrances to the building (rather than a single entrance),
  2. The case where the top speed is not attained within one floor jump (or even two or three floor jumps),
  3. The case of unequal floor heights,
  4. The case of unequal floor population,
  5. The case of combinations of above special conditions.

2- The Round Trip Cycle of an Elevator

A single lift car circulates around a building during the uppeak traffic condition in the following cycle:

  1. The car opens its doors at the main terminal floor
  2. Passengers board the car;
  3. The doors close.
  4. The car then runs to the first stopping floor going through periods of acceleration, travelling at rated speed, deceleration and levelling. (Travel at rated speed may not occur if the inter-floor distance is too small.)
  5. At the first stopping floor, the doors open and one or more passengers alight;
  6. The doors close. This sequence continues until the highest stopping floor is reached.
  7. In the highest stopping floor, after the doors have closed, the car is considered to make an express run to the main terminal, thus completing the round trip. 

Based on this cycle, we can define The Round Trip Time (RTT) as follows:

It is the time in seconds for a single car trip around a building from the time the car doors open at the main terminal, until the doors reopen, when the car has returned to the main terminal floor, after its trip around the building.

In the next article, we will explain the Round Trip Time (RTT) in detail.

Important Traffic Design Calculations

Now, we are going to learn how to get the most efficient and economic traffic design solution by using one of the following methods:

  1. The conventional design method,
  2. The Iterative Balance Method.

And before explaining the above two methods, we need to understand and learn how to perform the following important Traffic design calculations:

  1. Calculation of the Number of Round Trips for a Single Car,
  2. Estimation of Population,
  3. Calculation of the Average Number of Passengers per Trip (P),
  4. Calculation of the Uppeak Handling Capacity (UPPHC),
  5. Calculation of the Waiting Interval (Passenger Waiting Time),
  6. Calculation of The percentage population served (%POP),
  7. Estimation of Arrival Rate,
  8. Calculation of the Round Trip Time RTT,
  9. Estimation of quality of service.

1- Calculation of the Number of Round Trips for a Single Car

To calculate how many round trips a single lift car can complete during the peak 5-minute period, equation#1 will be used.

Number of round trips for a single car = 5 minutes / RTT = 300 / RTT         equation#1

2- Estimation of Population

The number of passengers that can use the lift will be comprised of one or more of the following passenger data sets:
  1. The number of passengers boarding from specific floors
  2. The number of passengers alighting at specific floors
  3. The traffic mode ie. unidirectional or multidirectional
  4. The transfer times for passengers entering and leaving cars
  5. Passenger actions.

Since we are dealing only with the uppeak traffic, these data are simplified to be:

  • Point (1) will be: passengers only load at the lobby.
  • Point (2) will be: passengers never alight at the lobby.
  • Point (3) will be: the traffic is unidirectional ie: travel is up the building.
  • Point (4) will be: the passenger transfer times will generally be “brisk”.
  • Point (5) will be: there is little opportunity for passengers to misbehave.

To determine the number of passengers, who will board and what their demand will be, depends on the building population.

2.1 The factors affecting the estimation of building population

The number of occupants will vary according to:
  1. The purpose of the building (residential, commercial or institutional),
  2. The quality of the accommodation,
  3. The type of occupancy (in the case of office buildings, the type of tenancy).

The size of the intended population should be obtained from the building owner or proposed occupier, if possible (and in writing). However, it may be that the population size is not available, or the building is a speculative one, when an estimation must be made.

A- Purpose of a Building
The buildings are generally defined as:
  1. Residential: where people live, eg: blocks of flats
  2. Commercial: where people work, eg: offices
  3. Institutional: where people receive a service, eg: a hospital.

B- The quality of the accommodation
The more prestigious the building, eg: a head office, the more space is available to each occupant.

C- The type of occupancy

There are three main types of tenancy:

1- Diversified tenancy:
It is a building occupancy condition, where no single tenant occupies more than a single floor and no more than one quarter of the tenants of the building are engaged in the same type of business activity.

2- Mixed tenancy:
It anticipates the possibility of multi-floor occupancy by a single tenant or multiple tenants with the same business activity.

3- Single tenancy:
It is a building occupancy condition where a single tenant occupies a substantial portion or zone of the building (say 80%). The single tenancy situation can present a severe traffic design condition. The group handling capacity with such occupancy can be high (about 14%) for calculation purposes. And some single-tenant insurance companies, government entities, or utilities, with large numbers of clerical workers can have handling capacity requirements of substantially more than 15% of the population in five minutes, if they operate a fixed starting time regime. In these cases, it would be important to establish this demand from the prospective building owner before carrying out any calculations.

2.2 Main Terminal Population

The main terminal population is not normally included in the design population for the following reasons:

  1. The main terminal floor is the bottom terminal for the lift group and passengers on this level walk to their work places.
  2. The main terminal floor is occupied in total by a bank or is a retail space. Employees of these types of businesses generally start work later than businesses occupying the majority of the building and, therefore, do not affect the major morning office building traffic peak.
  3. The main terminal in a building with underground or off-site /street parking is served by a separate bank of lifts. In this situation, the persons working on the main terminal floor and parking in the underground level would ride this separate bank of lifts to that floor. In the case of traffic from off-site parking or ground level drop-off traffic (taxis, etc.), they would walk directly into the main terminal floor and on to their work place.

The main terminal population may be included in the building population in the following situation. There are underground parking levels served by the same group of lifts that serve the upper floors of the building. Thus persons who work on the main terminal floor and who park in the underground levels would use this group of lifts.

2.3 Usable Area and Rentable area

Most population estimates start from knowledge of the net usable area, ie: the area which can be usefully occupied and which excludes circulation space (stairs, corridors, waiting areas), structural intrusions (steelwork, space heating, architectural features, etc.), toilet facilities, cleaners’ areas, etc.
The American National Standard ANSI Z65.1–1980 “Standard Method for Measuring Floor Area in Office Buildings” gives a useful guide to calculating areas in office buildings. It defines two important terms:

Rentable area Definition:
  • The rentable area is determined by measuring between the inside finished surfaces and/or dominant parts of permanent outside walls, excluding any major vertical penetrations (stairs, lift shafts, flues, pipe shafts, ducts and their enclosing walls). No deductions are made for any columns and projections necessary to the building. 
  • Rentable area generally remains fixed for the life of the building and is used to calculate rents.

Usable area Definition:
  • The usable area is determined by measuring between the finished surfaces of the office side of corridors and/or other permanent walls and/or the dominant parts of outside permanent walls and/or the center of partitions within the rentable area. No deductions are made for any columns and projections necessary to the building.
  • Usable area indicates the actual occupiable area and is important in lift traffic design calculations. Usable area can vary during the life of the building as corridors, partitions, etc. are moved.

In most traffic design cases, we can calculate and project the building population from the following equation:

The estimated population = the usable area of the building / the area allocated per person (in m2)

Where architectural drawings are too schematic to make an accurate estimate of areas, one of the following approximate rule of thumb relationships may be used, when the gross area is known:

Rentable area=90–95% of gross area
Usable area=75–80% of gross area

or the relationship below if the rentable area is known:

Usable area=80–85% of rentable area


  • Sometime the term Net Internal Area (NIA) is used. This is basically the area from the inside surfaces of the external walls. No concession is given for penetrations.
  • Whenever traffic calculations are made it is important and advisable to indicate (in writing) which estimations have been utilized and that a check review of the initial study is made once the architectural drawings are developed to a point where an accurate usable area calculation can be made.
  • In some cases, the building population may be dictated by the Owner/Client. This is particularly true if the building is being designed for a known occupant.


Using rules of thumb above, what are the rentable and usable areas of

(a) a tall/slender building and
(b) a low/squat building, each having a gross area 5000 m2?


(a) This will have a large core compared to the footprint, but the occupants will always be close to a lift.
Rentable area=90% of gross area, ie: 4500 m2
Usable area=75% of gross area, ie: 3750 m2

(b) This will have a small core compared to the footprint, and the occupants may be far from a lift.
Rentable area=95% of gross area, ie: 4750 m2
Usable area=80% of gross area, ie: 4000 m2

2.4 Practical Population Estimations

Table-2 gives guidance for a variety of buildings based on surveys and experience of the population to be accommodated.

Building  type
Population estimate
1.5–1.9 persons/room
1.5–1.9 persons/bedroom
3.0 persons/bed space*
0.8–1.2 m2 net area/pupil
Office (multiple tenancy):
10–12 m2 net area/person
15–25 m2 net area/person
Office (single tenancy):
8–10 m2 net area/person
12–20 m2 net area/person
* Patient plus three others (doctors, nurses, porters, etc.).
Table-2: Estimation of population

3- Calculation of the Average Number of Passengers per Trip (P)

As each car has a defined rated car capacity (CC) that it can accommodate, but the number of passengers assumed to be carried on each trip is taken as 80% of rated car capacity. This does not mean cars are assumed to fill only to 80% of rated car capacity each trip but that the average load is 80% of rated car capacity.

Therefore, Industry practice assumes a car loading of 80% of rated capacity. Values less than 80% do not fully utilize the installation, and values above 80% quickly result in poor service times.

P = CC x 80/100                      equation#2

4- Calculation of the Uppeak Handling Capacity (UPPHC)

The handling capacity (UPPHC) of a lift system is the total number of passengers that it can transport in a period of 5 minutes during the uppeak traffic condition with a specified average car loading.
A period of 5 minutes for the handling capacity definition has achieved general acceptance as it lies between one hour and a reasonable average waiting time, typically 30 s.

Therefore the 5-minute handling capacity (UPPHC) for a single car is:

UPPHC = average number of passengers per trip x 300 / RTT       equation#3

So, from equation#2:

UPPHC = P x 300/RTT                        equation#4

In installations with more than one car, Equations #4 become:

UPPHC = P x L x 300/RTT       equation#5

Where L is the number of lift cars.


  • The handling capacity of a lift installation indicates the quantity of service a lift system can provide. If the handling capacity of a lift system is too small, there will be lot of people queuing for the lifts during up peak. 
  • Also, the lift cars will have to go more round trips in order to clear off the queue. Thus system with too small handling capacity will degrade the quality of service.


5- Calculation of the Waiting Interval (Passenger Waiting Time)

Interval (INT) is the average time between successive lift car arrivals at the main terminal floor with cars loaded to any level.

With a single car the interval between successive arrivals is the round trip time.

Interval = RTT             equation#6 for a single car

However, where a lift system contains more than one car the interval becomes:

Interval = RTT / L                   equation#7 for L number of cars

Fig.3 illustrates the relationships between round trip and interval.
Fig.3: Relationship between round trip time and interval

Uppeak interval (UPPINT)

Uppeak interval (UPPINT) is the average time between successive lift car arrivals at the main terminal floor with cars loaded to 80% of rated car capacity during uppeak traffic conditions.

Equation#5 can now be rearranged, using Equation#7, to give the handling capacity of a group of L lifts:

UPPHC = P x 300/ UPPINT     equation#8


  • Passenger waiting times cannot be easily measured. Some designers, therefore, use the interval of car arrivals at the main terminal as an indication of service quality. 
  • The interval of car arrival of the lift system must be short enough so the handling capacity of the lift system will be suitable to the arrival rate of the passengers at the morning 5 minutes uppeak period.

6- Calculation of The percentage population served (%POP)

The percentage population served (%POP) is the number of passengers who arrive, at the main terminal of a building, for transportation to the upper floors over the worst 5 minute period expressed as a percentage of the total building population.

%POP = UPPHC x 100 / building population                       equation#9

uppeak handling capacity, uppeak interval and uppeak percentage population served, are the parameters most often quoted by a lift supplier.


A building is served by three lifts with a round trip time of 150 s. The building population is 400 persons and each car has a rated car capacity of 10 passengers. Calculate the uppeak interval, uppeak handling capacity and percentage population served.


From equation#7
Interval = RTT / L = 150 / 3 = 50 s

From equation#2

P = CC x 80/100 = 10 x 80 / 100 = 8 persons
Then, from equation#5

UPPHC = P x L x 300/RTT = 8 x 3 x 300 / 150 = 48 persons / 5 minutes

From equation#9
%POP = UPPHC x 100 / building population = 48 x 100 / 400 = 12%

In the next article, we will continue explaining other Important Traffic design calculations. Please, keep following.

The previous and related articles are listed in below table:
Subject Of Previous Article
Applicable Standards and Codes Used In This Course,
The Need for Lifts,
The Efficient Elevator Design Solution
Parts of Elevator System Design Process
Overview of Elevator Design and Supply Chain Process.

The Concept of Traffic Planning,
The (4) Methods of Traffic Design Calculations,
Principles of Interior Building Circulation:
A- Efficiency of Interior Circulation

B- Human Factors

C- Circulation and Handling Capacity Factors:
Corridor handling capacity,
Portal handling capacity,
Stairway handling capacity,
Escalator handling capacity,

Passenger Conveyors (Moving Walkways and Ramps) handling capacity,
Lifts Handling Capacity.
D- Location And Arrangement Of Transportation Facilities

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