Interfloor traffic is passenger movement between upper floors that never touches the main lobby, and it's the traffic pattern most likely to be missed by an analysis that only checks the morning peak. A design that clears the up-peak comfortably can still produce long waits at 11am or 2pm if interfloor movement wasn't tested separately.
Most elevator analysis work, understandably, treats the morning up-peak as the governing case, because it's the sharpest, most concentrated demand a lift group will ever face. That focus is correct as far as it goes. The risk is stopping there, because a design that passes on up-peak numbers alone can still disappoint occupants at other times of day.
What the Traffic Mix Actually Looks Like
Since 2020, ISO 8100-32, the international standard for lift traffic planning, has set out defined traffic mixes for exactly this reason. A typical office morning peak is modelled as roughly 85% incoming, 10% outgoing, and 5% interfloor traffic, with a required handling capacity of at least 12% of population in five minutes. Lunchtime traffic looks completely different: closer to a 40% incoming, 40% outgoing, 20% interfloor split, with its own, separately defined handling capacity and waiting time targets. A design validated only against the first mix has never actually been tested against the second.
It's worth being precise about why this gap exists rather than treating it as an oversight. Up-peak traffic is easy to reason about: everyone starts at the lobby and travels up, so the worst case is obvious and the formula is well established. Interfloor traffic has no single worst case in the same way, because it depends on which floors are talking to which other floors, and that pattern is specific to how a building is actually used rather than to its shape. A generic calculation has no way to know that, which is exactly why it tends to get simplified away.
Why This Matters More in Some Buildings Than Others
Single-tenant offices with meeting-heavy cultures, buildings with a shared restaurant or gym floor, and multi-tenant towers with cross-floor collaboration all generate meaningfully more interfloor movement than a conventional office. In these buildings, interfloor traffic isn't a minor background pattern; it can rival the up-peak as the more demanding design case. INFORMS, the operations research institute, has published analysis showing how passenger origin-destination data collected across a full day, not just the morning peak, changes what a lift group actually needs to handle.
None of this is a criticism of up-peak analysis itself. It remains the right governing case for most conventional offices, and getting it right is still the foundation of a good design. The point is narrower: up-peak passing is necessary, not sufficient, and treating it as the whole answer is where otherwise careful analysis quietly falls short.
How This Gets Tested Properly
Static calculation methods can approximate interfloor demand with a traffic-mix factor, but they struggle to capture the compounding effect of a lift already carrying passengers when an interfloor call arrives partway through its run. Simulation handles this more naturally, modelling the full population's movement across a working day and showing where interfloor demand actually creates the worst wait, rather than assuming it will always be secondary to up-peak.
This is one of the clearer cases where traffic analysis software earns its cost over a spreadsheet formula: it lets a design team see the lunchtime and interfloor scenarios side by side with the morning peak, rather than assuming the hardest test is always the first one.
The practical consequence shows up months after handover, when interfloor-heavy periods produce complaints that a design report never predicted. By then, the fix options are the same expensive ones that follow any late-discovered sizing gap: destination-control retrofits, revised dispatch logic if the controller allows it, or living with the queues. None of that is necessary if the interfloor case was tested alongside up-peak in the first place, at a point in the programme when the answer could still change the design.
What to Ask For
A traffic study that reports only up-peak interval and handling capacity has answered half the question. Ask specifically whether lunchtime and interfloor scenarios were tested, and against what mix. If a building has known collaborative floors, a shared amenity level, or a mixed-use programme, that request isn't optional; it's the difference between a lift group that works at 9am and one that works all day.
