The proposed Canton-to-Hong Kong super highway may not come soon, but what was once one of the most tense international frontiers on earth is now being rapidly erased.

In a development that also could have significance for Taiwan, still a major sore sport as the United States and China begin full diplomatic ties, China has granted Hong Kong new business concessions, opened new transit routes and considerably relaxed controls on the movement of its own people across the once heavily armed border.

Regularly scheduled air jetliners and hydrofils have begun to cross the border between Hong Kong and China in a wink of an eye, with no checkpoints and only quick customs inspections at Hong Kong or this southern Chinese city.

Chinese officials in Hong Kong have told local journalists that they even plan to allow through railroad traffic across the famous border bridge at Lowu, where travelers until now have had to leave their Hong Kong train, carry their baggage across a small wooden bridge and board a Chinese train.

Perhaps most importantly for Peking's future contacts with still hostile businessmen on Taiwan and in other overseas Chinese communities, mainland officials and overseas Chinese in Hong Kong have begun to open up many more business ventures on each others' territory.

With China about to offer unusual multiple reentry visas to overseas Chinese from Hong Kong and a Hong Kong television comedy show planning a broadcast from Canton the once doubtful future of this British-run center of Capitalism stuck on the coast of a major Communist power now seems as bright as its subtropical sun.

"These developments have been building for some time, but they are accelerating enormously now, and we have to be pleased, one Hong Kong official said recently.

Many officials here and in Hong Kong now see the British territory as a model for the sort of friendly cooperation between radically different political and economic systems they think could grow in the distant future between the mainland and Nationalist Chinese-held Taiwan.

Only 11 years ago, the border areas and downtown streets of Hong Kong seethed with fear and hostility as Chinese loyal to Peking rioted against the British government. During that period of tension, several people were killed, many pro-Peking demonstrators were jailed, and property values hit bottom as investors tried to sell fast before the mainland Army swept in to end the struggle.

Instead, three months ago the head of the Hong Kong office of China's New China News Agency - considered Peking's unofficial ambassador there - rose at a reception in honor of China's national day and toasted Queen Elizabeth II. Raising a glass along with director Wang Kuang was Hong Kong Governor Sir Murray Maclehose. It was the first time for a Hong Kong governor even to attend such a function.

One result of the decline of tension and fading of the border has been a growing flood of legal immigrants leaving here for Hong Kong. The British authorities would like to limit this. But they find it difficult to do so in the current era of good feeling. More than 9,000 legal immigrants arrived in Hong Kong in November, a monthly record in a year in which immigration is certain to more than double the 1977 total of 26,000.

Peking is acting to increase goodwill among overseas Chinese in Hong Kong, on Taiwan and elsewhere in the world by cutting red tape for their relatives living in the mainland who wish to leave. At the same time, Peking is apparently doing little to stop disenchanted peasants and workers living near this capital of Kwang-tung Province from crossing the border illegally.Even when caught and returned to China by Hong Kong police, the runaways usually find some way to return to the border and try again.

Along with the immigrants, however, Hong Kong also receives a huge flow of vital water, food, petroleum and other goods from China, so British officials are unlikely to complain too long and hard about China's lax border procedures. For the first eight months of 1978, total Chinese trade with Hong Kong jumped 25 percent to about $1.3 billion. Almost all of that was Chinese goods purchased by Hong Kong, but there were significant increases in Chinese purchases of Hong Kong goods such as textile yarn, fabrics and plastics.

At the semiannual trade fair here in November, Chinese officials listened while one Hong Kong-based trader proposed building a superhighway to speed the daily transport of pigs, chickens, fuel and countless other goods across the border. The Chinese gave no indication such a plan was in the works, but officials in Hong Kong say checkpoint procedures for trucks that cross the border each day have been eased.

Sir Lawrence Kadoorie, chairman of China Light and Power Co. and one of Hong Kong's leading entrepreneurs, has suggested to the Chinese that they supply coal for a huge new power plant he is constructing in the New Territories, the largely rural part of Hong Kong close to the Chinese border. He suggested that the plant might pay for the coal in part by selling electricity back to China, where blackouts often hit parts of this city.

Such an exchange would, in fact, indicate an official Chinese decision to allow the New Territories to remain in British hands beyond 1997, when a 88-year-lease originally signed with China's last imperial dynasty runs out. Peking has officially repudiated the lease as the act of an imperial clique not sanctioned by the Chinese people, but it has given numerous assurances this year that it plans to take no acts - such as recovering the New Territories - that would imperil Hong Kong's economic future.

Businessmen with ties to Peking are expressing confidence in Hong Kong's future by paying large sums for choice commercial properties on Hong Kong island and Kowloon. China operates an extensive banking system in Hong Kong and has opened a chain of gasoline service stations.

In return, China has allowed Hong Kong overseas Chinese to resume owernership of buildings here and in other parts of Kwangtung seized when the Communists took over. Hong Kong businessmen are also setting up a number of factories and hotels here and in other parts of China as part of an ambitious program of joint ventures. Peking seems to feel most comfortable dealing with Hong Kong compatriots.

Analysts in Hong Kong wonder if the advent of full diplomatic relations between Peking and Washington may ease some of the suspicion China has felt toward Hong Kong as a center of cynical U.S. China watchers and rumormongers. Many American journalists and diplomats who write about China are expected to move to Peking, leaving Hong Kong as a center for American businessmen, looking for a piece of the growing China-Hong Kong boom.