  Siran constructed infinite families of k-crossing-critical graphs for every
k=>3 and Kochol constructed such families of simple graphs for every k=>2.
Richter and Thomassen argued that, for any given k>=1 and r>=6, there are only
finitely many simple k-crossing-critical graphs with minimum degree r. Salazar
observed that the same argument implies such a conclusion for simple
k-crossing-critical graphs of prescribed average degree r>6. He established
existence of infinite families of simple k-crossing-critical graphs with any
prescribed rational average degree r in [4,6) for infinitely many k and asked
about their existence for r in (3,4). The question was partially settled by
Pinontoan and Richter, who answered it positively for r in (7/2,4).
  The present contribution uses two new constructions of crossing critical
simple graphs along with the one developed by Pinontoan and Richter to unify
these results and to answer Salazar's question by the following statement: for
every rational number r in (3,6) there exists an integer N_r, such that, for
any k>N_r, there exists an infinite family of simple 3-connected
crossing-critical graphs with average degree r and crossing number k. Moreover,
a universal lower bound on k applies for rational numbers in any closed
interval I in (3,6).
