Frequencies for first 10,000 rows of nhs variable in accident dataset :

   National |
    Highway |
     System |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      6,993       69.93       69.93
          1 |      2,984       29.84       99.77
          9 |         23        0.23      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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