We found this really interesting article on the shapes and layouts of road signs on Ralf Hermanns' OpenType info blog.
Dr. Raoul Bell conducted a study in which he found that the type of sign significantly influences the time we need to find the target. When all information was presented on one sign and without any separation, the targets could be found much faster. Bell argues that we perceive these objects as groups and only one group can have our attention at a time.
When the information is split into several groups we need to shift our attention from one group to the next, and therefore need more time to perform this task.
It is interesting to see how Australian road signage complies with this paradigm:
ID/Lab mostly applies the following principles to directional signage, which is based on direction & grouping:
The layout and order in which the information on a sign is listed has a significant effect on how quickly people can find the information they need. People like to be able to read signs quickly, and most people will only give a couple of seconds to find the information. As a result, most directional signs should have limited information.
On larger directional signs, lines sharing a common direction should be grouped together. They become easier to scan and reduce the number of arrows required, enhancing clarity. Signs like directories should be arranged in alphabetical order, to avoid visitors having to read every line to find their department.
Grouping Messages Vertically: Arrows are linked to the first line of message groupings which are then ranged left or right according to the direction indicated by the arrow. Arrows to direct straight ahead normally appear on the left except when indicating a route with a bias to the right.
Messages ranged left are grouped above messages ranged right in the order shown below. Where space is restricted, a panel with all ranged left messages can be placed alongside a panel of all ranged right messages.