Why car park signs should lie

Published in Traffic Engineering & Control,

March 2009, vol. 50, pag. 2-3

Last changes June 2009.

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A complex sign

The information needed to park your car
To decide where to park, a car driver arriving at Schiphol Airport has to study two signs containing more than ten words and pictograms in order to select between P1 and P2 (see figure at the right). Theoretically this will take approximately 2.33 seconds. When driving at 50 km/h the focus of the driver is away from the road for 32.86 metres. In practice extra time is needed to understand the P1 and P2 information and to process the other information presented (three traffic signs, arrival, and car rental).

Car park information on the roads to Amsterdam/Schiphol Airport

There are three dynamic displays. In this case two of the ‘park here information displays tell the driver ‘you cannot park here (‘vol, Dutch for ‘full). Since dynamic technology is used on this panel, the information ‘you cannot park here need not be shown, it is just additional information that the driver has to read and which delays his decision.

Concealing this option would give the driver approximately 16 metres extra to select one of two lanes. This is his main driving task. Concealing this option would also reduce the number of reading and understanding errors. So far, we have only used common sense. Using cognitive psychological principles, parking efficiency can be increased and costs can be decreased even more.

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What does 104 mean?

The car park signs in the photograph have the character P, an arrow and a number. In this picture the number of 104 is presented for Previum P2. What could this number possibly be telling us?

Count up?
The number might indicate the number of cars in the car park. That might sound stupid because the number of free spaces is more interesting to car drivers. But when computers are used it is common practice to present the content of the database directly to the user. Public transport, for instance, frequently presents time of departure, which is in the database, and does not count down to time of departure.

Count down?
On the Schiphol parking sign the number presented for the count is green. This colour might mean it is a ‘counting down number. We would prefer more clarity in this situation because many users are inexperienced. The word ‘spaces could be used but is ambiguous because it is not clear whether it refers to spaces which are available or occupied. ‘Available spaces is clear but takes up a lot of space. Just ‘free is shorter and clear too. Free could be omitted when counting down has become an international standard. This will reduce misunderstanding, time for perception and costs.

Free only?
The sign tells the driver that he can park 104 cars and not 105. In Holland one driver, one car is common practice. We suggest no number is necessary but just the word ‘Free tells the driver all he needs to know.

Free when I arrive?
Of course saying that 100 places are available informs the driver that finding a place is very likely. However, reliability of getting a place should be elaborated in another way. The greater the distance between the information ‘Free spaces and the car park, the less reliable this information is. The real question for the car driver is: ‘Will there be one place when I arrive. As most of these systems have memory and computers, a good estimation should be possible. If the play in the theatre above the car park will end in a few minutes a sign at 15 minutes drive from the car park indicating that there are no vacancies is telling the truth but the information is unreliable. So should the signs lie?

Free in a few minutes?
The information ‘Full does not help the driver find a parking space. The only option offered to him is to continue driving to the car park, hope to find a parking space to park before he arrives at the car park and hope that the queue at the car park is not too long. The sign should rather give information that enables the driver to choose a strategy. He should be informed about the trend.

Getting fuller, there is a queue, hurry up or take another option if you see one.

Getting emptier, small queue, slow down and your waiting time will be less.

Of course, many drivers will think: ‘Ah, FULL, but vacancy expected in 5 minutes. So there will be several drivers for one vacancy in 5 minutes. However, the system knows that several drivers will come and are prepared to wait for 5 minutes. When the system expects a vacancy in 5 minutes it should say: ‘Vacancy in 20 minutes. Experience will teach the car park operator when the system has to say that there are vacancies. The benefits will be that drivers will have a place when the system says so and that they will learn to trust the ‘lying system.

Conclusion:
An intelligent full/free light on a sign is better than a stupid dynamic number. Better means better for the car driver and earlier and incorrect but reliable information on the chance of parking his car. Better also means better use of the car park: more cars and more income.

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Selecting a car park

One car park is full
The Schiphol Airport board presents two car parks, one is ‘Full (Dutch: vol). Simply not showing this option would save the driver a lot of reading, interpreting and decision work as suggested earlier.

Both have free spaces
Suppose both have free spaces? On this board the difference between P1 and P2 is just the numeral: the number 1 and 2. It is unclear which one is the best for the driver.

Which one is closest to my destination?
In this case all car drivers have the same destination and parking close to the terminal is the best choice. One could inform them about that. However, this increases the amount of information on the sign. Alternatively, one could hide the car park not close to the terminal when the closest one has free places. The size of the sign and dynamic technology used on the signs could be reduced to one letter P and one dynamic arrow pointing left or right. When both car parks are full a third arrow might point to a car park at a greater distance.

The current sign does not seem to provide that information. Experienced users might know that the car park at a greater distance to the terminal has free spaces when the arrow points to the closest one. They might prefer the more distant car park because that one is closer to their office. Directing car drivers to one option only, even when two options are available, gives the opera tor a tool to increase the efficiency of the flow of visitors. He could reduce traffic on particular routes or increase the use of unpopular car parks. In this airport case, the car park operator might manipulate the use of the car parks by suggesting that the one closest is full. In doing so he can keep free places for a group of visitors paying extra for a guaranteed place in the car park close to the terminal.

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Which one is cheapest?

Another way of controlling traffic and car park use is pricing. Pricing is already in use for simple choices like long stay and car parks at greater distances from the destination. Computers and dynamic signing could be used in a more complex manner to control traffic and car park use better.

This might make the pricing system more complicated. However, a complex pricing system should be no problem for computers. For drivers the complexity can be reduced to words or signs for ‘expensive or ‘low cost. When these words or signs are used in a standard way the information load for car drivers will be low.

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Large volumes of information and dynamic technology are expensive and suggest that the operator has state-of-the-art equipment and is willing to spend money on informing his customers. Also suppliers of this technology will gladly deliver complex signs.

From a car drivers point of view it is better that computers compute, take decisions about where to park and illuminate one of three arrows the car driver should follow. Few car drivers will realise that such a car park operator is ahead of the game and providing a far better product at a lower price.

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What does 104 mean?

Selecting a car park

Which one is cheapest?

More applied cognitive psychology for public design:

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