By Steve Gorman

LOS ANGELES (Reuters) - One upside of the worst U.S. economic slump since the Great Depression is that with fewer motorists on the road, the nation's highways are less congested and in better shape, a study said on Thursday.

The condition of urban interstate highways and rural primary roads both showed significant gains in 2008, the latest year for which complete figures were available, according to the Reason Foundation, a Los Angeles-based think tank.

Urban interstates and rural primary roads, accounting for most of the 140,000 highway miles covered in the group's 19th annual highway report, also were found to be in their best condition since 1993.

Moreover, urban interstate congestion and the rate of highway deaths per miles driven continued to decline, mirroring trends for improved road and bridge conditions during recent years, the study found.

Much of the improvement reflects increased spending on state-administered roads, which has nearly tripled since 1984 and jumped 8 percent from 2007 to 2008 alone.

But the recession was cited as a key factor as well.

"During 2008, automobile travel fell about 3.5 percent from 2007 levels, reducing congestion and fatality rates and slowing road deterioration," the report said.

Federal dollars allotted for economic "stimulus" projects have contributed 22 percent more resources for highway improvements starting in late 2008 and continuing into 2009 and 2010, the study added.

However, the economic downturn has dampened federal and state fuel tax revenues, putting the pace of future highway projects into doubt, said Adrian Moore, a transportation economist and foundation vice president who co-authored the report.

"Seeing things improve is good news, but I don't think people should get too excited," he said.

The study was based on data reported by state transportation agencies to the U.S. Federal Highway Administration, Moore said.

While the overall portion of urban interstates classified as congested fell below 50 percent for the first time since 2000, motorists in California -- the most populous state -- remained stuck in the heaviest city freeway traffic.

California also ranked next to worst, after Hawaii, in the percentage of its urban interstate pavement surfaces deemed to be in poor condition, nearly 24 percent, and as the worst for rural interstate roughness.

Conversely, California's highway fatality rate ranked among the best, at 13th, with just 1.05 deaths per 100 million vehicle miles. By comparison, Massachusetts posted the lowest fatality rate, 0.67, and Montana the highest, 2.12 per 100 million miles driven. The national average was 1.25.

(Editing by Eric Beech)