Interstate 78

Approaching the I-78 interchange on I-81 north in Union Township, Pennsylvania

I-78 begins at a directional-T interchange with I-81 in Union Township, Pennsylvania about 25 miles (40 km) northeast of Harrisburg. Near the east end of the county, at exit 8, U.S. Route 22 (US 22) merges with I-78, running concurrently for the next 43 miles (69 km) through Berks and Lehigh Counties.[3]

At exit 51, in Upper Macungie Township, US 22 leaves the highway. Drivers on I-78 eastbound must use this exit to access I-476 (Northeast Extension of the Pennsylvania Turnpike), and westbound travelers must use exit 53 (northbound Pennsylvania Route 309 (PA 309) and then westbound US 22. From exits 53 to 60, I-78 runs concurrently with PA 309. The six-lane overlap bypasses the city of Allentown to the south and crosses South Mountain.[4][5]

I-78 eastbound past the PA 33 interchange in Lower Saucon Township

At exit 60 (A-B going westbound), PA 309 yields south to Quakertown.[6] Approximately 6 miles (9.7 km) east, there is an interchange between PA 412 and I-78 in Hellertown, serving Bethlehem and Lehigh University. At exit 71, PA 33 reaches its southern terminus at a trumpet interchange. PA 33 traverses the Pocono Mountains as it enters Bangor and crosses I-80. The final exit on I-78 in Pennsylvania is Morgan Hill Road, providing access to PA 611 and Easton. I-78 then crosses the Interstate 78 Toll Bridge and enters the state of New Jersey.

After the Interstate 78 Toll Bridge, I-78 enters New Jersey as the Phillipsburg–Newark Expressway.[7] The road begins by running parallel with County Route 642 (CR 642) in the town of Alpha. At 3.94 miles (6.34 km), a partial cloverleaf interchange brings together US 22, Route 122, and Route 173 with I-78 in Phillipsburg.[8] US 22 now runs concurrently with I-78 for the next 15 miles (24 km). Going westbound, exit 4 leaves to the right for CR 637 and Warren Glen. The next exit, Exit 6, is for CR 632 in Bloomsbury. However the route number is not signed on I-78. Exit 7 is the first of several eastbound exits for Route 173. This one is located in Bloomsbury as Route 173 begins to parallel the Interstate. 4 miles (6.4 km) later, exit 11 leaves to the right as another exit for Route 173. CR 614 also is located off the exit. Exit 12, westbound is for Route 173 again. However, Exit 12 eastbound is for a frontage road parallelling I-78.

I-78 westbound in Warren Township, New Jersey

Exit 13 is only westbound and is another exit for Route 173. Nearby the exit, going eastbound, the frontage road merges in.[8] Exit 15 is for Route 173 and CR 513 in Franklin Township. Exit 17 is for Route 31 in Clinton. In the town of Annandale, US 22 leaves I-78 at exit 18. US 22 continues towards Bound Brook and Union County. At exit 20, CR 639 intersects. CR 639 heads towards the Round Valley Recreational Area. Exit 24 is for CR 523 towards Oldwick. At exit 29, I-287, US 202, and US 206 interchange with I-78 in Bedminster. At this point, in Somerset County, exits 33, 36 and 40 are for county routes in Warren Township. At exit 41, I-78 enters Union County,[8] and then passes through the Watchung Reservation, where land bridges cross over the highway to allow wildlife to transit safely. At exit 45, CR 527 intersects after paralleling for some time. West of exit 48, I-78 splits into express and local highways. Exit 48 is for Route 24 in Springfield. Exit 49A is for one of Route 24's spur routes, Route 124. Exit 52 is for the Garden State Parkway in Union. At Exits 57 and 58, Route 21, US 1, US 9, and US 22 intersect I-78. The exit provides access to Newark Liberty International Airport.

I-78 eastbound at the Newark Bay Bridge

East of exit 58 at the eastern tip of Newark, I-78 becomes the Newark Bay Extension of the New Jersey Turnpike. Past the first toll plaza, I-78 has an interchange with I-95 (The New Jersey Turnpike) and crosses Newark Bay via the Newark Bay Bridge.[8] The first exit, 14A, is for Route 440 in Bayonne. The Liberty State Park can be reached by taking Exit 14B. Exit 14C is the final numbered exit, providing access to the Liberty Science Center. Route 139 runs concurrently with I-78 as it approaches the Holland Tunnel and enters New York.

I-78's length in New York is only 0.5 miles (0.80 km)—half of the Holland Tunnel and the egress-only roundabout immediately beyond the end of the tunnel. The route was planned to run east and north through New York City to end at I-95 in the Bronx, but sections of the planned route, including the Lower Manhattan Expressway, were cancelled.

I-78 at the Holland Tunnel

In New York City, I-78 continues through the limited access egress-only roundabout known as the Saint John's Rotary. The five separate exits from the Rotary are assigned numbers—exits 1 to 5—in counterclockwise order. The last one—and the logical continuation east—is exit 5, Canal Street. Under the original plans, I-78 was to continue across Manhattan as the Lower Manhattan Expressway onto the Williamsburg Bridge, and then beyond I-278 on the never-built Bushwick Expressway through Brooklyn into Queens near the John F. Kennedy International Airport. A section of I-78 at the airport was built as the Nassau Expressway, later I-878 and now New York State Route 878 (NY 878), though most of the westbound side was never built. East of the airport, I-78 would have turned north on the Clearview Expressway (built north of Hillside Avenue in Queens and now I-295), run across the Throgs Neck Bridge, and forked into two spurs, ending at I-95 via the Throgs Neck Expressway (now I-695) and the Bruckner Interchange via the Cross Bronx Expressway (now part of I-295).[9]