The end of next month will bring the 50th anniversary of the opening of the Auckland Harbour Bridge, a symbol of the city and the key to its northward development ever since. Quite how the city will mark the occasion remains unclear and has become needlessly contentious.

The bridge's administrative body, the Government's Transport Agency, is thought to have not yet completed any plans for the day, May 30, but it has ruled out one obvious idea: allowing people to walk or bike over the bridge for a few hours that day. That was how Aucklanders celebrated the construction of the bridge shortly before its opening in 1959 and many of today's citizens would no doubt relish an opportunity to do the same, particularly as the anniversary falls on a Saturday.

So why is the Transport Agency against it? Cost, it says. Somehow it calculates that closing the bridge to traffic for six hours would cost $1.2 million. This figure seems to reflect the expense of catering to an estimated 320,000 pedestrians, a third of the region's population. If the event is likely to attract that many it would be worth it. Sadly, the real reason for the agency's reluctance is probably the risk of encouraging one group of citizens who want the bridge to cater for foot and cycle traffic every day. The "Getacross Campaign" is planning to mark the anniversary with an unauthorised march and cycle crossing of the bridge on Sunday, May 24. It believes that a big turnout will help its case for walking and cycle ways to be added to the bridge's flanks. It would prove nothing of the kind.

It would be an opportunity the vast majority would seize for its novelty value. They might enjoy the stroll, or the bike ride, for itself, or as something they were proud to have done, particularly since the opportunity might not come again. Enjoyment of a rare experience is seldom evidence of a desire to do the same thing every day.

Morning and evening commuting by bicycle or on foot across the harbour bridge would be very different from joining a festive crowd for a crossing on a sunny (hopefully) Saturday afternoon. The Transport Agency should credit citizens with the good sense to see the difference. It need not fear that those who take advantage of such an opportunity would lend unthinking support to a campaign for a costly clip-on facility that, when it came to the point, few would use regularly.

The agency's fears will not have been helped by the Auckland Regional Council's request for an approved "trial" in place of the May 24 protest. The agency agreed to allow it if the council accepted a $30,000 bill for traffic management and related costs. Council chairman Mike Lee says it has not got the money, but if $30,000 is the true cost of opening the bridge for a few celebratory hours, it can surely be done.

Auckland's 50-year-old icon deserves better than a ragtag protest march to mark its milestone. The organisers say they will not be celebrating the bridge's presence as they see it as a barrier to reducing Aucklanders' reliance on cars. Buses, of course, have been using the bridge for as long as cars. The amenity cannot be blamed for the fact that most people plainly prefer the convenience and independence of personal transport.

The bridge at 50 is facing the usual challenges of maintenance and projected need. But with the clip-ons added a few years into its life, and the "tidal" lane controls permitted by a moveable barrier installed in 1990, the venerable coat-hanger is handling modern traffic volumes much better than some other motorway bridges in Auckland. A companion crossing, alongside and preferably under-water, is not yet a pressing need. The city will rely on its familiar bridge for many years yet. Let's celebrate it with an anniversary walk.