Touchscreens for Many Hands

A big touchscreen isn't a big phone. People use their whole arm, they stand at different heights, and they often arrive in groups. Most of what we know from mobile design carries over badly.

Reach comes first

Nielsen Norman Group's piece on very large touchscreens points out that a 72-inch screen is about 380 times larger than an average smartphone, so targets have to allow for arm motion, reach and the user's height. A menu in the top corner may simply be out of reach for many people, and a control at the bottom may mean bending down.

Keep the controls in a comfortable band and let the content above it be for looking at. Then check it with someone shorter than you, and with someone in a wheelchair.

Targets: minimums are only minimums

The standards give you a floor. WCAG 2.5.8 asks for at least 24 by 24 CSS pixels at Level AA, and Apple's guidance gives 44x44 points as the default on iOS and iPadOS.

Those are for devices held in the hand. On a wall-sized screen, go much bigger. Fitts's law explains why: the time to reach a target depends on its size and distance, and error rates fall as targets get bigger. Distances on a big screen are long, so size has to make up for it. Also remember that CSS pixels on a kiosk may not map to anything like the size you'd expect, so measure in millimetres on the real glass.

Touch points aren't people

A spec sheet can say a lot of touch points. At Tabadul's Gitex stand, we had multiple seamless 55-inch displays, each with capacitive touch for up to 100 simultaneous touch points. But the hardware only tells you that many fingers can be seen. Your interface decides whether several people can do anything useful at once.

Avoid modal dialogs that lock the whole screen, global menus that one person can change for everyone, and timeouts that wipe out what somebody else was reading.

Give people their own space

There isn't much solid guidance on multi-user digital tables. One useful study is older and looked at traditional tables: Scott, Carpendale and Inkpen found that people working together use personal, group and storage territories. It's a reasonable starting point. Put things in front of the person who asked for them, and keep shared content in the shared space.

Expect the unexpected

Sleeves, bags and children all count as touches. Build recovery states and a clean reset, and make the idle screen work from across the aisle. This is what our interactive apps work is about.