Entities that are subject to sections 9-500.04, 28-474, 49-474.01 and 49-573 shall comply with the following time schedule for retrofitting their existing diesel vehicles:

1. 1. Twenty-five percent of the diesel fleet vehicles by December 31, 1998.
2. 2. Forty percent of the diesel fleet vehicles by December 31, 1999.
3. 3. Sixty percent of the diesel fleet vehicles by December 31, 2000.
4. 4. Eighty percent of the diesel fleet vehicles by December 31, 2001.
5. 5. One hundred percent of the diesel fleet vehicles by December 31, 2002.