The number of people who live in their vehicles because they can't find affordable housing is on the rise, even though the practice is illegal in many U.S. cities.

The number of people residing in campers and other vehicles surged 46 percent over the past year, a recent homeless census in Seattle's King County, Washington found. The problem is "exploding" in cities with expensive housing markets, including Los Angeles, Portland and San Francisco, according to Governing magazine.

The problem of vehicle residency is national in scope, although its impact may be more "acutely felt in urban areas where space is more limited," said Sara Rankin, an assistant professor law at Seattle University and the director of Homeless Rights Advocacy Project, in an email to CBS MoneyWatch.

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Challenges abound for people who live in their vehicles, ranging from racking up parking tickets to finding a safe place to park and shower, advocates say.

A recent survey by the National Law Center on Homelessness and Poverty (NLCHP), which tracks policies in 187 cities, found the number of prohibitions against vehicle residency has more than doubled during the last decade.

"Much like outdoor camping and sleeping bans, city-wide restrictions on living in vehicles may leave no lawful place where homeless people may live in a community," NLCHP said in a recent report. "Bans that permit vehicle impoundment, or that result in impoundment flowing from unpaid tickets or other enforcement of such bans, can cause homeless people to lose their shelter, transportation, and personal belongings in one fell swoop – with no realistic option to retrieve or replace them."

It's a frequent problem for the youth and veterans who turn to the Volunteers of America in Illinois for help, impacting perhaps one-third of the organization's clients, said Airielle Macool-Cunningham, manager of veteran's support services.

"People don't consider themselves to be living in their car if they are only doing it for a couple of nights here and there, and so they are not self-reporting that," she said. "Since our climate is a lot colder, they don't typically stay in their cars long term unless it's the summer months."

Stephanie Monroe, managing director of Children Youth & Family Services at Volunteers of America, Dakotas, tells a similar story. At least 25 percent of the non-profit's Sioux Falls clients have lived in their vehicles at some point, even during winter's sub-freezing temperatures.

"Many of our communities don't have formal shelter services," she said in an interview. "It can lead to individuals resorting to living in their cars or other vehicles."

Homelessness rose last year, marking its first increase since 2010, according to the U.S. Department of Housing and Urban Development. About one-third of the homeless population was described as "unsheltered," which includes people living on the streets and in their vehicles. HUD's data doesn't provide more specific information.

A fair number of the "vehicular homeless" in Silicon Valley are employed but are unable to find affordable housing, as the Associated Press noted last year. Lines of RVs can be found near the headquarters of tech heavyweights such as Apple, Google and Hewlett-Packard. Nationwide, extremely low-income renters are facing a shortage of 7.2 million rental homes, according to the National Low Income Housing Coalition.

"A lot of times, once you lose your home it can spiral downwards from there," said Megan Hustings, interim director of the National Coalition for the Homeless, in an interview. "We have seen people living in their cars anywhere from a couple of weeks to months to years. "