DALLAS — Williamson County, Tex., the ninth fastest growing county in the U.S., has gained a one-notch upgrade to AAA from Standard & Poor’s, analysts said.

“The raised rating is based on our view of the county’s consistent strong economic expansion and its proven ability to manage growth,” said analyst Kate Choban. “Further supporting the rating is the county’s maintenance of strong finances.”

Standard & Poor’s general obligation rating now matches that of Fitch Ratings and Moody’s Investors Service.

The upgrade came as the county prepared to issue $35 million of pass-through toll revenue and limited-tax bonds. Officials believe they will need nearly $600 million of bonds to address growth-driven road improvements and extensions in the long-range transportation plan and finance the county’s share of right-of-way acquisition costs for state highways.

The county has $80 million remaining from a $228 million voter authorization for road improvements. It has no plans to hold any additional bond elections.

Proceeds of the upcoming issue will be used to finance the construction of and improvements to U.S. Highways 70 and 183, Farm to Market roads 1660, 2338 and 3405, and State Highway 29. The project is part of the pass-through toll program of the Texas Department of Transportation.

The county has experienced explosive population and property tax base growth in the past five years, partly due to its close access to Austin.  Most of the growth has come in the towns of Round Rock, headquarters to computer maker Dell Inc., and the bedroom community of Leander, recently connected to downtown Austin by a new commuter rail line.

According to the most recent census data, Williamson is the ninth fastest growing county in the nation and is typical of the other affluent counties on the outskirts of major cities. According to 2010 estimates, the population has grown to 410,686. To the north, Bell County, which includes the massive Fort Hood Army Post, is also growing rapidly.

“While not immune from the effects of the economic downturn, the Austin metropolitan employment base is broad and continues to outperform much of the country,” according to Fitch. “The county’s own economy continues to diversify, with unemployment rates below state and national averages.”

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