North Dakota does the best job maintaining its roads and bridges and New Jersey has the worst-performing, least cost-effective highway system in the nation, according to an annual Reason Foundation study
that measures each state's road conditions and expenditures. Massachusetts' roads are the safest; Montana's are the deadliest. 

Across the country, 24.1 percent of bridges are deficient or functionally obsolete. In Rhode Island, over 53 percent of bridges are deficient. At our current rate of repair it will take 62 years for today's deficient bridges to be brought up to date.California has the most traffic congestion: 83 percent of its urban interstates are congested. But other states are becoming increasingly gridlocked: 18 states report at least half of their urban interstates are jammed. Even South Dakota has traffic congestion now, according to the study.
 
The Reason Foundation's 17th Annual Report on the Performance of State Highway Systems by University of North Carolina at Charlotte Emeritus Transportation Professor David Hartgen, released July 31, measures the condition of all state-owned roads and highways from 1984 to 2006.  The study calculates the effectiveness and performance of each state in 12 different categories, including pavement condition, bridge condition, traffic fatalities, congestion, highway maintenance costs, and administrative costs.  Here is a sample of some of the report's data:
 
California - 44th
California has 18,251 miles of state-owned highway system, slightly larger than the national average. California ranked 44th in the overall state highway performance and cost-effectiveness ratings, the same as last year. Its best ratings were for deficient bridges (9th - 17.72 percent of the state's bridges are deemed structurally deficient or functionally obsolete), fatality rate (20th) and narrow rural primary pavements (23rd). However, California has the highest percent urban interstate congested in the entire nation, 83.3 percent. It also performed poorly on administrative disbursements per mile of responsibility (49th), urban interstate in poor condition (48th), receipts per mile of responsibility (47th), total disbursements per mile of responsibility (47th), capital/bridge disbursements per mile of responsibility (45th), maintenance disbursements per mile of responsibility (44th) and rural interstate condition (41st). California's total disbursements increased about 33 percent from 2005 to 2006, yet its system performance remained essentially unchanged.

For more information, go to the following links:
 
Full Study (.pdf)
http://www.reason.org/ps369.pdf
 
State-by-State Rankings
http://reason.org/ps369table02.pdf
 
Deficient Bridges Rankings
http://reason.org/ps369table14.pdf
 
Press Release, Google Map, Detailed Spreadsheets and More
http://reason.org/ps369/
 
Reason Foundation is a free market think tank that has advised the last four presidential administrations on transportation issues.