Leslie Graves and Geoff Pallay, researchers at Ballotpedia, have published this report on the lack of voter choices on the ballot for state legislative races. The report says 32.7% of legislative districts have no contest between the two major parties this year.
Ballot Access News published a somewhat similar study in the October 1, 2010 print issue, and found 36.1% of the races lack a contest between a Democrat and Republican. The difference between the two studies is because of different treatment of multi-member districts. The Ballotpedia study considered a district to have a Democratic-Republican contest if it elects, for example, 5 members, and there are 5 Democrats and 1 Republican running. But the Ballot Access News study considered an example like that to lack Democratic-Republican competition for 4 seats.