From what I've been reading about US patents, once a patent has expired, it enters the public domain and can be freely used. The patent I was looking at was filed on 1/1999 and published in 10/2000. ...
US6148942 "Infant stroller safely propelled by DC electronic motor" According to Google Patents the 6148942 patent is status Lapsed and according to Public PAIR its status is Expired as of 2008. ...