The Rock 'n' Roller Coaster at the Disney resort in Florida features a frightening version of the L.A. freeway, where you go from zero to 60 miles per hour with the force of a supersonic F-14 Tomcat, accompanied by high-decibel heavy metal music by Aerosmith.

It's a ride unlike any you've ever experienced -- unless, of course, you count the one provided by the stock market this year.

After enduring several months of mostly negative volatility, a lot of investors would like to get off the ride. I don't know about you, but I'm getting nauseous.

Baby boomers who were smart enough to invest in target-date funds -- those no-brainer, one-size-fits-all vehicles that automatically shift to more conservative investments over time -- have escaped a lot of the tumult, right?

Not exactly. You can expect those with a date of 2020 or later to get hit hard, since they'll have a lot of equity exposure by design. But many with a target date of 2010 -- that's a little more than a year away -- have been reeling in this market environment, with losses ranging from 15 percent to more than 40 percent year to date. Eeeeeeeeeeeeee!

What gives? These funds are supposed to shed stocks and purchase bonds as the target date approaches, or at least that's what the fund marketing literature says. Yet some of these 2010 target-date funds have high stakes in stocks.

Close inspection of a 2010 fund
If you're invested in AllianceBernstein 2010 Retirement Strategy, for example, you would have nearly two-thirds of your portfolio exposed to stocks, 31 percent in bonds and 3 percent in cash equivalents, according to Morningstar. And your fund would have lost 37 percent in the 12 months through mid-November.

Ironically, AllianceBernstein published several research reports recently in which it characterized the target-date funds of leading competitors as too conservative and not properly diversified, according to Forbes.com.

"Some shops like AllianceBernstein, which has some of the most aggressive target-date funds around, have done a lot of work on this," says Morningstar fund analyst Greg Carlson. "They've used Monte Carlo simulations. There's a lot of research backing what they do."

Monte Carlo simulations involve running different portfolios through thousands of market scenarios to see which combination of equities, bonds and money market funds would improve the probability that a portfolio would last as long as you do.

"The high-level rationale is that there is a trade-off between the risk of losing principal, and a concern about the risk of not having enough accumulated to last through retirement or keep up with inflation," says Nevin Adams, editor in chief of plansponsor.com, which advises benefits and retirement decision-makers.