1915-1931, possibly the peak of board track racing, due to the popularity, the number of race cars and race car drivers, the number of manufacturers making cars that were competitive, cheap labor to build the tracks, cheap inexpensive lumber, and the number of board tracks to race at. The economic times were flush in the late 20's, things were good, the depression hadn't hit yet.
Motorcycles were racing board tracks first, and in 1911 to 1924 were in heavy competition with big purses to the winners. The tracks were 3/4 to 2 miles, and were made of 2x4s... the speeds kept getting faster, the engine sizes smaller, and the number of riders and spectators dieing never stopped. The bikes had no brakes, the tracks had no safety rails, and the spectators had no caution. A good source of 1910-1926 cycle racing at http://thevintagent.blogspot.com/2008/12/board-track-racing-on-film.html
the above cars are going 70 mph. Not bad in your car, on a highway or interstate... now imagine that 70 mph in a 1930 car, with no power steering, skinny tires, on a surface that looks like the below. Dangerous
by 1931 there were 24 board tracks around America. Beverly Hills, Atlantic City, Uniontown PA and the Bronx were some, probably the big dollar ones http://en.wikipedia.org/wiki/Board_track_racing
notice the size of the shadows next to that guys feet. It's a good comparison in size, and I think a couple inches of height irregulaity, or gap, would not be a bad guess
anoter shot of the rough conditions that the steering of those 1920's cars, and the 1920's tires had to deal with. Not good. They were driving 140 mph in the above event