"We're shocked how many helmets have disappeared," a city official said.

But an inspection on Tuesday showed 47 percent of the 858 helmets were missing.

The city started providing helmets at 30 public bike rental stations in Yeouido last Friday in preparation for their mandatory use starting Sept. 28.

Half the bicycle helmets the Seoul Metropolitan Government began lending cyclists free of charge four days ago have already disappeared.

The helmets are put in the bikes' carrier baskets or in separate containers, and users should have returned them that way. But many simply made off with them.

City officials initially considered putting tracking chips on the helmets, but the cost of W1.2 billion a year put them off (US$1=W1,135). Instead they decided to trust people's better instincts, and were sorely disappointed.

Now the city may scrap the helmets altogether.

This is not the first time that a free public sharing project has flopped. Attempts to share books, umbrellas and medicine have failed because there has always been a significant proportion of thieves.

Seoul Metro started lending around 1,300 books at 13 subway stations for free but stopped after just two years because most of the books were never returned. It also started providing band aids and sanitary towels at 35 subway stations but scrapped them after just six months because people scooped them up and hoarded them.

Gangnam District Office began providing 450 umbrellas at 22 ward offices in July last year, but a staffer said, "We now have only 30 to 40 umbrellas left."

Experts attribute the failure to a lack of civic spirit in an increasingly fragmented megacity.

Yoon In-jin at Korea University said, "Many people still consider products offered as public services freebies. Our society is still in transition to becoming a trust-based society, so it's probably better to charge a small amount or check ID when providing such services."