Frequencies for a_vroll variable in veh_aux dataset :

   Rollover |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |         41        6.21        6.21
          2 |        619       93.79      100.00
------------+-----------------------------------
      Total |        660      100.00

by Jean Roth , jroth@nber.org , 18 Apr 2016