London's Congestion Charge Web Interface leaves Much to the Imagination

Today the congestion charge went live. If you do not live in the UK's capital and haven't heard the indignant cries of drivers and commuters, this may have passed you by. However, as the largest scheme of its kind in the world, both the scheme and the means of its implementation are in the hot seat of evaluation.

Drivers into the centre may pay by phone, text or by going to the website. It is this last interface that has received scrutiny from South Bank University's Fintan Culwin, a man working on the edge of the zone. He makes the following usability points in response to the problems he found:

1. Soak and capacity testing of a system should be completed in good time before it goes live. Less than 1 week before congestion charging starts, when the system could be considered live, response times are unacceptable.

2. Respect the users customisation of their environment. The browser preferences for text size are ignored by the system. (10 pt is the dominant font size.)

3. Prevent obvious errors from happening. The use of a pull down menu would reduce date/month confusion.

4. Make effective use of screen space. Having the calendar permanently displayed would make use of space that is not used for anything else and provide an alternative, less confusing and more natural input mechanism.

5. Indicate input formats clearly. The provision of a 'dd/mm/yyyy' indicator on the form would reduce the probability of input errors.

6. Provide clear and consistent feedback. Error messages offset from the centre of attention in small font and low contrast colours are likely to be missed. Error messages are for the benefit of users not developers, so diagnostic errors should be removed.

7. Provide clear and consistent feedback. The use of a dialog to communicate with the user in some circumstances and an on-screen message in others makes it even more likely that the on-screen messages will not be attended to.

8. Error messages should indicate the cause and/or the cure for the error. Although 29/02/2003 is an 'Invalid date' explaining why would be a little more helpful.

9. Maintain consistency on the interface. Removing an error message from the interface once the user has attended to it prevents a possible visible contradiction.

10. Respect established conventions. The greying out of a component to indicate its unavailability is a widely accepted convention. The use of an underline to indicate active status is appropriate for a hyperlink but should always be visible, not transiently visible and partly obscured when the cursor passes over it.

11. Less is more. The use of a number of different colour schemes on the calendar to indicate different reasons for unavailability is confusing and unnecessary.

12. Provide timely feedback. The server round trip required by the calendar could and should be removed to provide faster user confirmation.

13. Selection is better than input. As the interface has sufficient space for the calendar component it could be used exclusively to select the required date.

Associated Link: Congestion Charge Site

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