The association "Media-Analyse" publishes an annual report with usage figures for the most important Austrian media.
The methodology is a large sample (14,500) of respondents, who are asked when they last passed by a billboard, a City Light, respectively.

In 2008, 84% of the Austrians older than 14 years passed billboards and 46% passed City Lights within seven days.
The development is moderately rising, largely due to increasing mobility.

More information  can be found >here (german website)