There's plenty of street parking (metered) in the neighborhoods surrounding the campus. You should easily be able to find a spot near the New Haven Greens (centrally located park) or on the streets with shops near The Greens and between State Street - and then walk around the campus.

Cut and pasted the following info from Yale University website: The Mead Visitor Center is the front door of the University and welcomes its visitors from around the world. It is located at 149 Elm Street New Haven, CT 06511, across the street from the New Haven Green. Drop by for information about Yale or to take a guided tour led by Yale College undergraduates. Groups of 10 or more visiting the campus are required to schedule a private tour. Private tours and foreign language tours are available for a fee by appointment. Guided Tours Mon–Fri: 10:30am & 2pm Sat-Sun: 1:30pm

Just like Paul said.

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If you go alone, you can only visit he University from the outside I think. With the guide you enter at least to the gardens and see things from inside. If you go alone, also you miss lots of interesting information. The guided tour is free, so I recommend you take it

No purtroppo è solo in inglese! Se hai altre domande ti rispondo volentieri!

You can find info about booking a tour in Italian in the university page visitorcenter.yale.edu /tours