All across‑road cul­verts and entrance cul­vert shall com­ply with the spec­i­fi­ca­tions. No such pipe cul­verts, how­ev­er, shall be less than fif­teen (15) inch­es in diam­e­ter unless writ­ten approval is giv­en by the Coun­ty Engi­neer for a reduced size, a min­i­mum of twelve (12) inch­es in diam­e­ter. The sizes of all drainage struc­tures shall be com­put­ed by using “Talbot’s For­mu­la” for runoff. The design, instal­la­tion and con­struc­tion of all drainage struc­tures shall be com­put­ed by using con­struc­tion of all drainage struc­tures shall be sub­ject to the approval of the­Coun­tyEngi­neer and the Road Dis­trict High­way Com­mis­sion­er and/or Illi­nois Depart­ment of Transportation.