How even apparently simple and well understood interfaces can contain subtle error traps
Back in the day learner drivers of the new M113 Armoured Personnel Carrier or APC were found to be repeatedly veering off the road or into oncoming traffic when trying to carry out an emergency stop. In some circumstances learners would also accelerate while trying to perform an emergency stop. The M113 was examined and no mechanical design fault or failure was found. A mystery, and needless to say the army brass were somewhat concerned.
In the M113 driver control is executed using two control levers between the drivers legs, these control the left and right tracks of the vehicle. To go straight ahead both levers are pushed forward, to turn one lever is moved forward while the other is in the neutral position (for a neutral turn) while to stop, the driver pulls back on both steering levers at the same time. The two hand control levers driver interface is one that was first developed for civilian bulldozers then applied to military vehicles so there was nothing new or developmental in its nature. however the speed at which a bulldozer and the early tanks operated at was much less than that of a M113, because the M113 were also designed for operation on roads.
This driving task is a classic example of what Rasmussen (1983) called ‘skill’ based behaviour, that is highly automated behaviour requiring little conscious thought. But as it turns out physiologically humans don’t react with both hands at the same instant or with the same amount of force, so the driver’s instinctive skill based response in an emergency stop results in a differential input into the control levers and the APC veering off course. This was unnoticed in slower vehicles but very noticeable, and alarming, in the case of the M113.
And the sudden accelerations? The reason is simply that when trying to execute an emergency stop, drivers instinctively leveraged their force input by pressing on the floor with both feet, one of which, was on the accelerator. When driving a bulldozer at slow speed on a construction site the slow operating speed allows time for detection and correction by the operator. But when the vehicle is moving at high speed on a roadway and negotiating it’s way through traffic things are different, the tolerance for human error much less and an error traps emerge.
The take home
This was a ‘simple’ and quite well understood interface, yet within it were hidden two different error traps that emerged when the operational context changed. Changes in the ecology (1) of a user interface can have interesting and unexpected consequences, even for supposedly simple user interfaces and tasks. For more complex interfaces and systems doubly so.
Rasmussen, J. (1983). Skills, rules, knowledge; signals, signs, and symbols, and other distinctions in human performance models. IEEE Transactions on Systems, Man and Cybernetics, 13, 257-266.
1. Ecological interface design is a modern approach to interface design focusing upon the ecology or context of work tasks.