For women, commuting across this ancient Islamic city has long been as easy as hopping into a minibus or climbing on the back of a motorcycle taxi. Both are cheap and readily available. Even if some female passengers found it unsettling to be so near strange men, who might make lewd comments or press their bodies close, such was the price of efficient transport.

But the days of casual travel are ending for the women of Kano, a bustling trading center of about 500,000 in northern Nigeria. Government officials, determined to halt what they see as the decline of public morality, are banning women from all but a handful of Kano's motorcycle taxis and are requiring them to sit in the back of public minibuses.

It is the next logical step, officials said, in their effort to bring the strict Islamic legal code, or sharia, to Kano, which is in one of 12 states in northern Nigeria where Islamic law holds sway to varying degrees. The remaining 24 states, and the federal capital, Abuja, have a mix of religions and are governed by secular laws.

Since 2000, authorities across northern Nigeria have sought to reestablish traditional sharia rules disrupted in the 20th century by British colonialism and post-colonial political struggles, including floggings for drinking alcohol, amputations of hands for stealing and death by stoning for adultery. The harshest of these penalties have rarely been carried out, but a broader campaign toward regulating behavior -- especially in relations between men and women -- has taken hold.

Underlying the move toward sharia is a growing concern that life is changing too fast in Kano. Traders hawk DVDs of often-lewd Hollywood movies. Residents who can afford satellite television can get an eyeful of dancing, scantily dressed women. And some young women are choosing not to wear the traditional head coverings or long, loosely fitting robes preferred by their elders.

The new transit strategy, for which the government has bought a small fleet of gender-restricted vehicles, has met initially with widespread approval. Men say they believe the virtue of women is better protected when the sexes sit separately. Women say the new minibuses are more comfortable and private. Sitting in the back, they say, frees them from the potentially amorous gazes of men sitting behind.

Zabbaatu Auwal, a woman carrying a toddler, boarded one of the new minibuses at a hectic, smoggy depot. The vehicle was emblazoned with the words "A Daidaita Sahu" or "Be Orderly," in Hausa, one of the four most widely used local languages beside English. Taking a seat in the back row, she praised the new system.

"I don't have to mix with men, which is a source of discomfort to us," explained Auwal, 27, as she held her 2-year-old son.

However, the second phase of the transportation policy, which will include the ban on women riding motorcycle taxis, threatens to be far less popular. In the next few weeks, police will begin fining motorcycle drivers caught carrying women who are not their relatives. Drivers licenses also may be suspended. And gender-based seating restrictions will extend to all commercial minibuses, even those that are privately owned.

That will leave women with far fewer choices for getting to work or school or going shopping. If the seats designated for women on a minibus are full, they will have to wait for the next bus or one of the new single-sex vehicles. The government has purchased 176 motorcycles and 500 three-wheeled vehicles with covered seating areas that are physically separated from the driver. Together, though, they represent a fraction of the tens of thousands of public transport vehicles that ply Kano's streets.

This is a dense, fast-paced city, with a centuries-old historic quarter whose narrow streets are not accessible to minibuses. Aisha Lawal, a 19-year-old student, said she will have difficulty making her twice-weekly visits to see her grandparents if the vast majority of motorcycle taxis are prohibited from carrying her. She predicted resistance from women.

On a sidewalk a few blocks away, Miriam Muhammed, 24, prepared to climb onto a motorcycle taxi. "We don't want this," she said of the new system. "Goodness, I'll be frustrated."

Many men argue that the new rules were brought on by Kano's chaotic traffic. The motorcycle taxis, which are noisy and spew streams of bluish exhaust, whip in and out of preposterously small pockets between moving cars.

The Hausa word for a motorcycle taxi is achaba, meaning "to make a silly mistake." Serious injuries among drivers have become so common in recent years that the city's biggest public hospital has an area known as the "achaba ward."

But it is the contact between a man and woman -- there is no way to ride on an achaba without legs and torsos touching -- that makes them particularly alarming to this traditional community. Polygamy is common here, but sexual contact outside marriage, or even the rumor of it, can generate tremendous shame and condemnation, not just for a woman but for her family as well.

Sule Yau Sule, a senior government spokesman in Kano, said some of the motorcycle drivers pick up women for the thrill of feeling them sitting close behind.

Avoiding such problems in the future, he said, is worth the investment, even in a place with a sagging economy and a dangerously overloaded road system. So far, the government has spent about $2.1 million on the gender-restricted vehicles, which are leased to private drivers, and it plans to buy many more.

"It's the wish of the people," Sule said. "This is part of sharia."

The implementation of sharia has run into practical problems before, and throughout the north, few of the most serious penalties have been carried out. Nobody has been stoned to death for adultery. An initial spate of amputations for stealing stopped after three were performed, and most criminal cases are now directed to state criminal courts.

The sharia courts increasingly deal with marital matters, civil disputes and public drinking, an offense that typically is punished by lashes intended more to shame than to physically harm.

The "Be Orderly" slogan found on the new buses is from a broader campaign initiated by Kano's Gov. Mallam Ibrahim Shekarau. He warned on the program's Web site that "rules of behavior, civility and decency, which the people of Kano were renowned for, are on the decline."

Some men on crowded minibuses seem willing to make propositions they wouldn't have dared a few years ago, women passengers said.

"You can sit by a man who is not disciplined in his own mind," said Fatima Abdulkadir, 45. "They say things, or you see men trying to move toward you, trying to rub themselves against you, which is not permitted."

The new buses and three-wheeled vehicles may gradually eliminate that problem, but not the new motorcycles. Though they are restricted to carrying only women, the drivers will continue to be men, and their female passengers will continue to encounter a degree of physical contact widely regarded here as inappropriate.

One obvious solution -- hiring female drivers -- runs into another sensitive gender issue. Every bus, every motorcycle and the vast majority of private cars in Kano are driven by men. Few expect that to change.

"The traffic is mad," said Ahmad Kofar-Naisa, 47, a bus driver, dismissing the idea of female drivers. "Women can hardly cope. Even for a man it's hard."

In the next few weeks, police in Kano, a city in one of Nigeria's 12 Islamic states, will fine motorcycle drivers caught carrying women who are not their relatives. Driver's licenses may be suspended. The government has bought 500 women-only buses, a fraction of the city's tens of thousands of public vehicles. Many women say the new minibuses are more comfortable and private.