Originally carried out by use of manual calculations and statistical tables lift traffic analysis has, essentially, remained unchanged in its approach since the 1920’s. The introduction of computerised traffic analysis in the mid 1970’s however began a new era in the sophistication and speed of analysing lift traffic performance. It also enabled much more complex variations in floor populations, entrance/exit locations and control system types to be analysed quickly and accurately.
Lift traffic analyses for planning and comparative purposes is based upon the performance of a lift system (a given group of lifts or just a single lift) during a theoretical 5 minute period of peak traffic.
The performance of the lift service in this 5 minute peak period has two distinct measures. These are its potential quality (related to waiting time) and quantity (related to numbers of passengers handled). These theoretical performance characteristics can then be compared to industry benchmarks. Alternatively the performance can be compared between an existing lift system versus a modernised lift system. These measures can be more clearly defined as follows:
- Quality of Service – often measured by the time between consecutive lifts arriving at the main lobby and known as Average Interval (AI) or by the Average Waiting Time (AWT) – both measured in seconds. If we divide the 5-minute (300 s) peak period by the Average Interval we get the number of car departures in the peak 5-minute period.
- Quantity of Service – measured by how many people can be moved in a given type of traffic in 5 minutes. If we multiply the number of car departures within 5 minute period by the number of passengers in the car we can calculate how many people have been moved. Known as the 5-minute Handling Capacity it is usually expressed as a percentage [%] of the population served by the given group of lifts.
These measures of performance are directly influenced by the following:
- The working population of the building or part of the building served by the group of lifts.
- The number of lifts (including their capacity, speed and performance characteristics).
- The predominant traffic flow being simulated i.e. one way or two way.
- The lift control system utilised (conventional or destination).
- The number of floors served (including the total travel distance of the lifts).
- Where users enter and exit the building.
Let’s look at each of these influencing factors in turn.
This is the best estimate of the maximum number of people that might be present in the building or the part of the building served by a given group of lifts. See separate tutorial on Lift Performance Criteria for further guidance.
Number of Lifts
As well as the actual number of lifts their capacity (kg/number of persons), speed and performance in terms of acceleration, jerk and door width/door operating times all impact the traffic performance of the lift or group of lifts being analysed.
Predominant Traffic Flow
The peak 5 minute period of traffic can be during what is termed one way or two way traffic meaning passengers are predominantly moving in one direction (into or out of the building) or, in the case of two way traffic, moving more or less in equal quantities in both directions (into and out of the building).
For some buildings we may wish to analyse peak traffic during just two way peak periods, for others we may wish to analyse both peak periods, e.g. in an office building the one way morning peak period of up peak traffic as building users enter the building and the two way peak period during lunchtime as building users are both entering and leaving the building.
The table below defines the general approach to which traffic types are analysed in each type of building:
* Depends upon what type of building the car park is related to
Lift Control System
For most building types including hotels, residential, car parks and retail developments the lift control system generally adopted is what is termed conventional collective control with two landing push buttons to register both “up” and “down” direction hall calls at intermediate floors.
For office buildings intending to employ three or more lifts in one group the preference would normally be to adopt a destination hall call control system. This is where lift users book their destination floor using a keypad, touchscreen or secure card in the lobby and are told which lift will take them to their destination floor. There are no destination floor buttons inside the lift car. This latter control system can enable considerable improvements in system efficiency to be delivered thereby reducing the number, speed and capacity of lifts that might otherwise be required with conventional control.
Number of Floors Served
Any given lift or group of lifts will serve a fixed number of floors (in some instances it may be that not all lifts serve the same number of floors). The number of floors served, the inter-floor distance and therefore the total travel distance of the lifts from the lowest to the highest floor served all need to be accurately included in the base data for the traffic analysis to be performed accurately.
As a very rough rule of thumb lift speed is usually optimal using a figure of 1/20 (0.05) the total travel distance, e.g. for lifts travelling up to 100 m a speed of up to 5.0 m/s might be appropriate. This rule however breaks down at both very short and very long travel distances.
Entrance and Exit Floors
Where users enter and exit the building (including whether there are particular user attractions within the building itself, e.g. a staff restaurant) is also a very important factor. This is indicated by placing an entrance bias to the lift system. Most usually it is 100% at the main lobby but may vary if there is more than one entrance to the building or for other reasons.
Further reading: Lift Performance Criteria