Vision and perception
The cone of vision: where a screen does and does not stand out
We are far from seeing everything equally sharply. Straight ahead our vision is crystal clear; off to the side it quickly turns vague. That sharp zone, the cone of vision, determines whether a sign along the road is noticed at all.
Look straight ahead and try, without moving your eyes, to read something positioned right off to the side. You can’t, can you? Straight ahead you see sharply, but to the side everything turns blurry.
That sharp zone in front of your eyes is called the cone of vision. For a sign along the road this is hugely important: if the sign is within the driver’s cone of vision, it stands out. If it is too far to the side, the driver simply does not see it.
The cone of vision is the area in front of the eyes where vision is sharpest. It is linked to the fovea and the rapid fall-off of rods past about 20° of field angle. Research on motorists, the target audience of most outdoor signs, shows that signs within the optimal cone of vision can be noticed, and those outside it cannot.
How big is that cone?
Different studies give different values. Traffic research generally places the optimal cone of vision between about 10° and 35° around the line of sight; some sources recommend placement within 10° of the line of sight, others within an ‘optimal’ field of 15°. The shared conclusion: the closer to the line of sight, the greater the chance that the sign is noticed.
First stand out, then read
A sign must first stand out (conspicuity) before it can be read. Standing out depends on many factors, but placement is the most important. The zone in which a sign is optimally visible is called the placement zone or ‘lateral offset’. Only after that do character height and viewing distance come into play.
The fovea covers only ~2% of the field of view, yet via the occipital lobe it supplies most of the consciously processed image information. That is why conspicuity (detectability) is highly placement-dependent: a sign outside the cone of vision competes with the much lower peripheral resolution and is easily overlooked.
Placement zone and lateral offset
Because the cone of vision consists of two right-angled triangles, the permissible lateral distance (Sign Placement Zone or lateral offset) is calculated with the tangent function of the half-cone angle over the viewing distance. For each speed and character height this yields a maximum lateral offset, as given in the viewing-distance table. The cone is a three-dimensional concept: both horizontal and vertical deviation count.
Detection versus legibility
Full visibility of a sign is the sum of detection (conspicuity) and legibility. A perfectly legible sign that hangs outside the cone of vision scores poorly on detection and therefore on effective visibility. Placement and character height must be optimised together.