In the Constitutional law of the United States, a forum is a property that is open to public expression and assembly.
In determining what is reasonable, the Court stated that "[the] crucial question is whether the manner of expression is basically incompatible with the normal activity of a particular place at a particular time."
A traditional public forum is where speech/expression is supported by the first amendment and when the government's ability to regulate speech is reduced like a sidewalk or state park.
An open forum made by the government can be barred from expression as long as there is not a retaliation or yearn to reduce certain speech.
Prior to the legal development of substantive due process, state governments had the authority to regulate speech in public places without regard to the First Amendment.
However, in 1939, Justice Owen Josephus Roberts stated that "use of the streets and public places has, from ancient times, been a part of the privileges [...] of citizens.