Jed Brown drives 100 miles each day to work between Pennsylvania and West Virginia. Cheaper gas is making his commute more manageable, but he doesn't expect the low prices to last. Uri Berliner/NPR hide caption

itoggle caption Uri Berliner/NPR

While people can often estimate how much they might need for 10 or 15 years of retirement, that calculation becomes more difficult for retirement that could last 20 years or more. Gary Waters/Ikon Images/Getty Images hide caption

itoggle caption Gary Waters/Ikon Images/Getty Images