The Talmud implies that one who did not know of the establishment of an eiruv techumin before Shabbat may not make use of it. However, it appears that to some degree this is the topic of debate among rishonim. The Rosh (Eiruvin VII, 13,) observes that the reason that one is required to know about the eiruv is because others cannot include him without his knowledge, as its limiting aspects are considered a liability (chov). However, if the individual has expressed his desire to walk in that direction, he thus makes it clear that he regards the eiruv as an asset (zekhut) and thus others can include him without his knowledge. This however is apparently not the view of Tosafot (46b, s.v. ein chavin). R. Menachem Segel Pollack (Resp. Chelek Levi, 125) adduces additional support for the lenient view of the Rosh, and rules accordingly.