The science of letter sizes and number of words on billboards

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

30 October 2022

STOP!

Before you start reading, start a timer and time how long (in seconds) it will take you to read it. Today’s article is 407 words long – including these introductory sentences…

It is estimated that the average reading speed in English is about 250 words per minute, or about 4 words per second. So, this article should take you about 100 seconds – one minute and a half, or thereabouts.

Now think about advertisement billboards erected along the highways. They are targeted at people in fast-moving vehicles, perhaps 80km/h to 100km/h. What is the maximum limit of the number of words that can be read before the vehicle has passed the board?

At, say, 90km/h, the vehicle covers 25m per second. So, if the billboard only appears the field of view from 25m away, the message should only be 4 words long. Anything more than that will not be read – a waste of ink!

If the advertiser wishes to have more words in the message, then the board must be erected at a location where it enters the field of view of the audience from farther away. And that’s not all: there is also the question of the size of the letters in the message.

Do this simple experiment: measure the height of this letter ‘I’ in millimeters and write it down somewhere. Now, find out how far from the newspaper (also in millimeters) your eyes needed to be when making that measurement. Divide this distance by the size of the letter ‘I’.

What’s your answer? For most people, it is between 200 and 300. For that reason, the font size of any written text should not be less than a 300th of the distance from the target reader. That is, if the reader will be 25m (2,500cm) away, then the letters in the message should not be smaller than 8cm tall.

Clearly, the farther away the billboard is, the larger the letters should be. However, since the board has a fixed size, the bigger the letters, the fewer the words it can fit. It appears like a self-regulating situation, but designers must always bear these limits in mind.

If the message is intended for drivers, the time available for reading is even shorter. I estimate it to be about a quarter of the total duration – looks at the advert for one second, then at the road ahead for 4s.

Now stop the timer. What’s your reading speed?