Long bridge at Gibraltar 'twixt Africa and Europe?

Is a fixed crossing of the Mediterranean at the Strait of Gibraltar a Utopian fantasy? A fixed crossing means a tunnel or bridge between Spain and Morocco at the narrowest point of the Mediterranean -- the 15-mile-wide Strait of Gibraltar.

Hardheaded men -- Americans, British, Spanish, and Moroccans -- at the African Highways Conference of the International Road Federation, just ended in Nairobi, say they are convinced that it can be done. Before them was a report by a committee of the International Road Federation.

The background is a powerful move by African and European states to increase the number of highways linking the two continents to stimulate intercontinental trade and tourism.

At present the Mediterranean is crossed by ferries and ships linking European and African ports.

The idea of a fixed road connection through the Strait of Gibraltar goes back to 1869 when a French engineer presented a plan for a tunnel. Since then Spanish engineers have examined the possibilities of a tunnel or great tube. In 1956 a Spanish engineer, Alfonso Pena Boeur, produced a project that included a bridge across the strait.

The crossing is deep, strong tidal currents are common, and the geology is complex. Three suggestions have been made:

* Bored tunnel -- A tunnel would have to be at least 300 meters below sea level. Elaborate ventilation equipment would have to be provided.

* Sunken tube -- Spring tides and currents would make positioning difficult. A sunken tube would probably be above sea-bed level and anchored for its entire length.

* Bridge -- Two 3,000-meter spans in the center, crossing the deepest parts. A model suggested is the suspension bridge at Verrazano Narrows at the entrance to New York Harbor in the United States. Morocco already is carrying out a preliminary assessment of the technical and financial implications of such a project.

The difficulties and the cost of a bridge, however, are recognized as prodigious, with the cost put at no less than $80 million a kilometer. With the minimum length likely to be 16 kilometers, the construction cost of a fixed link could not, according to experts, be less than $1.25 billion.

The concept of a fixed crossing between Europe and North Africa relates to the enormous amount of traffic that would be released from the difficulties and dangers of ferry transport.

In 1975 more than 1 million passengers and 12,000 vihicles crossed from Algeciras, Spain, to Tangier on the Moroccan side. These figures are known to have increased in recent years.

A new highway now is planned from Morocco via Tangier, Rabat, Casablanca, Marrakech, and Agadir through Mauretania and Senegal, with a linkup to the trans-African route, Lagos to Mombasa, already constructed in parts.

More than 70 roll-on, roll-off ferries now link the European continent with North Africa.

Although a tunnel or bridge could not possibly be built before the turn of the century, it is believed, the Nairobi conference decided to recommend that urgent technical studies of such a project be carried out.