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

   National |
    Highway |
     System |      Freq.     Percent        Cum.
------------+-----------------------------------
          0 |      7,061       70.61       70.61
          1 |      2,925       29.25       99.86
          9 |         14        0.14      100.00
------------+-----------------------------------
      Total |     10,000      100.00

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