QR codes on vehicles, windows and large signage

Size is a function of distance, and the ratio is simple

A workable rule is that the code's width should be roughly one tenth of the intended scanning distance. Read from two metres, it needs to be about twenty centimetres. Read from five metres, half a metre. Most failed outdoor codes are simply too small, having been sized to look proportionate in the design rather than to be scanned.

Moving vehicles are a bad host for codes

A code on the side of a van is unreadable while the van is moving and unreadable while the reader is moving, which covers most of the time it is visible. If you put one there, treat it as a code for parked situations and pair it with a spoken-style short link that someone can remember from a glance in traffic. The typed link will outperform the code substantially.

Glass adds glare and depth

A code in a shop window is behind a reflective surface, often with the street behind the reader mirrored across it. Mount codes low enough to be shot at an angle that avoids the sun, avoid glossy laminates, and consider placing the code on the outside of the glass where the environment allows.

Height matters as much as size

A code above eye level is photographed at a steep angle, which distorts the grid and defeats some scanners. Between waist and shoulder height is the reliable zone. On a large banner, put the code low even if the design would rather have it in the corner.

Label the code, and put a typed link beside it

A bare square on a wall asks passers-by to gamble. One line — "Scan for today's menu" or "Scan to book a viewing" — is the largest single improvement to scan rate. Adding the equivalent short link in text costs a few characters and captures everyone whose camera does not cooperate.

Give each physical site its own code

Ten branches sharing one code produce one number. Ten codes produce a ranking, and a ranking tells you which site placements work and which are behind a bin. Since the codes are dynamic short links, this costs nothing but a few minutes of setup.

Plan for weather and wear

Outdoor codes fade, peel and get scratched. Use a higher error-correction level for genuinely exposed placements, specify UV-stable printing, and schedule a physical check. A code that stopped working three months ago and nobody noticed is the normal outcome without a check in the calendar.