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There’s barely a day goes by where we don’t see a misplacement of the term ‘Interval’, be it in correspondence, in conversation or otherwise. Almost unbelievably, the most frequent offender is in modern specification documents: we’re talking employers requirements, brand standards and contract tender packages to name but a few. As you might imagine, text which becomes contractually binding is not a place to be mincing your words.

A Century of Elevator Traffic Analysis

To understand why this might be, a quick history lesson is required. Stick with me!

Classically, lift system performance was analysed using calculation alone (prior to the development of robust computer simulation tools). The calculation used is known as the ‘up-peak round trip time’ (UPRTT) as it evaluates the round trip time of a single lift with all of the traffic in the upwards direction, for a given handling capacity. This was based on initial work by Jones (1922) and Schroeder (1955) and was formalised by Barney and dos Santos (1985) in a manner that is widely accepted by the modern lift industry. Dividing the UPRTT result by the number of lifts in the group provides us with a value for ‘Interval’.

It’s important to understand that the concept of ‘waiting time’ was not possible to evaluate using an UPRTT, or indeed any of the enhancements or improvements that were made to that calculation in the years that followed, most notable of which by Peters (Formulae for the general case, 1990). Hence, ‘Interval’ was the best available means to link system performance to passenger experience, as it describes the average time between lift cars departing the home floor. It therefore became the de-facto means of laying down a marker for lift performance in the late 1980’s and remained as such until arguably the early 10’s, in the UK at least.

There you are, history doesn’t have to be boring!

Interval is a System Parameter

So what’s the actual problem with ‘Interval’ appearing in a performance spec in 2019? Seems like a pretty good measure of system performance, developed by some of the greatest mathematical minds the lift industry has ever seen, right? Well, yes. Precisely that – interval is a system parameter, it is not an accurate indicator of passenger experience. Using ‘Interval’ alone as a design parameter can mask poor (or even catastrophic) real world performance. Consider the following theoretical examples:

A low rise building with a single elevator and an extremely low intensity traffic flow. The round trip time of this lift could be infinite, as the system is effectively not being used. The Interval would be the same as the UPRTT, as there is only one lift in the group (Interval = UPRTT / lifts in group). If targeting a design interval of 40s, it wouldn’t be possible to demonstrate this by calculation unless more (phantom) traffic was added to the lift group to increase the car departures, or the quantity of lifts were increased. In reality passengers would experience a nominal waiting time with a single lift.

A high rise building with multiple elevators and extremely intense traffic. In this case, we need to visualise that the interval describes the frequency between loaded lift cars departing the home floor. Now imagine the morning up-peak traffic flow in an office and queues beginning to form in front of the lifts. No matter how long the queue and the associated length of waiting time, the interval would not increase past it’s maxima as full lift cars continue to depart at a given frequency. Some queueing is inevitable in modern buildings but interval alone provides no way for us as designers to evaluate this, potentially masking poor performance in reality.

Is There a Right Way?

It’s important to note that we generally commence our traffic studies by using a General Analysis or UPRTT calculation to test the theoretical handling capacity of the system. The lesson here is not that ‘Interval’ is wrong, but that it cannot be considered in isolation. Modern traffic analysis uses dynamic simulation tools, which are more powerful and sophisticated than the classical calculations, permitting us to evaluate lift systems on the basis of ‘average waiting time’. This allows us to interrogate passenger experience in addition to system response.

Our recommendation is that you set your requirements for lift performance in terms of ‘average waiting time’, not ‘Interval’. This minimises the risk of an overdesigned or underperforming installation. CIBSE Guide D 2015 lays down modern markers for acceptable waiting times, along with methodology for carrying out simulations to attain reliable values for average waiting time.

Illuminating the Path

At Vercon we maintain an up-to date, balanced view on industry standards relating to passenger lift traffic analysis, planning and design – complemented by a strong connection to the developmental history of these practices. If you want to understand more about this topic and how it can help add value to your scheme, we love making new friends – just give us a call.

In the first insight on the website, I thought I might look at the most human element of lift design; the wait from button press to car arrival (we consider this concept as a parameter called ‘average waiting time’ when carrying out a lift traffic analysis). Thankfully, unlike the subjects of Beckett’s famous play inferred in the title of this post, the average lift user knows exactly what they are waiting for and where they hope to travel when the lift arrives.

An Instant Age

With most systems, there is an inevitable period of waiting for a response. In a lift system, it is captive to an extent (unless the stairs are an easy option) and normally without distraction or the convenience of even a chair; this facilitates a potentially frustrating experience. Depending on the popularity of the route, the evolution into ‘queue’ can be swift…and without mercy.

The advent of the Information Age in the 1980’s (and the stratospheric technical progress we’ve see seen since) has sharpened the response time of many day to day queries and processes from weeks to hours and from hours to seconds. Remember when checking the price of a flight meant phoning a travel agent? When revising a drawing required a pen and a stencil? Nowadays we live in an instant age, an age where people may not be as prepared to wait as their parents might have been. Even those of you who would consider themselves patient – how long would you persist with a webpage that is too slow to load? Digital marketing studies have shown that latency of as little as 3 seconds is enough to repulse a prospective buyer on an e-commerce site.

Design: One size fits all?

Many of you will know that lift system performance is often targeted and evaluated on the prospective ‘average waiting time’. I’ll be sure to upload a jargon-busting post in the coming days, but in the mean time it’s safe to say this term is pretty self-explanatory. Now it’s hard to argue with waiting time as a design parameter; with the right simulation tools and the relevant expertise it’s a extremely reliable indicator of passenger experience. But it’s harder to see how a one-size-fits-all value for waiting time will suit every building user, and especially in our instant society, whether that value is really fast enough.

Luckily for us as designers, there are some robust industry benchmarks for waiting time that we can work to as a starting point. In the simplest terms, they lie in different ranges depending on building use. Sadly the individual psychology of different passengers can’t really be taken into account within a single building type, but the hope is that the ‘average’ passenger will be content with the design standards set. And what of our instant expectations in an instant age? Well, luckily both our understanding of passenger traffic flow and the speed / complexity of lift equipment has increased dramatically in the last 25 years, allowing us as designers to really minimise those infamous waiting times. Really, no type of building user across market sectors should have to wait (on average) more than 60 seconds, with the highest performing systems realising waiting times of 25 seconds or less.

The Law of Averages

But what does the reader think about this ‘average passenger’, bearing in mind they may or may not have been that passenger this very morning? Let me pose a puzzler; below are 4 types of building user – can you rank them in order of the time they are theoretically prepared to wait for a lift? 1 being the longest waiting time and 4 being the shortest. Answers are, of course, at the end of the article.

• A student in halls of residence leaving to visit the pub with his pals.
• A office employee taking the lift to their 17th floor desk on a Monday morning.
• A hotel guest, returning to their room after a delicious breakfast.
• A penthouse apartment resident, leaving for work in the morning.

Easy, right?!

Before you read the answers, remember that things are not always as they appear on the surface. For instance, the lift performance in an office building will be focused on productivity and efficient people flow, not how quickly an employee wants to get to work. It is natural that stakeholder requirements for building efficiency will not always fully align with passenger preference, but we should as designers aim to minimise this division where practical.

Mirror Mirror

So in summary, although waiting periods are naturally subjective and inherently linked to passenger psychology, we use our experience and the guidance that exists within our industry to set realistic requirements and deliver a holistic solution for a given scheme. It’s our passion at Vercon to delve deeper; that’s what sets us apart.

One last thing about that ‘average’ passenger. Did you know that they are prepared to wait twice as long for a lift than they usually would if there is a full length mirror in the lift lobby? Now the evidence for this is anecdotal at best, and we cant say we’ve tested the theory, but I would draw your attention to the uptake of the ‘selfie’ and the rising popularity of Instagram; perhaps mirrors have a viable future in the management of passenger psychology after all!

Answers: (Based on CIBSE Guide D: 2015) 1A, 2D, 3C, 4B