Interstate 78

Pennsylvania

I-78 begins at a directional-T interchange with I-81 in Union Township, Lebanon County, 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).

At exit 51, in Upper Macungie Township, US 22 leaves the highway. Passengers traveling 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.

At exit 60 (A-B going westbound), PA 309 south leaves for Quakertown. 6 miles (9.7 km) later, there is an interchange between PA 412 and I-78 in Hellertown. PA 412 also goes to Bethlehem and Lehigh University. At mile marker 71, PA 33 intersects with I-78 at a trumpet interchange. PA 33 traverses the Pocono Mountains and goes to Bangor and I-80. The final exit on I-78 in Pennsylvania is for Morgan Hill Road, which goes to PA 611 and Easton. I-78 then crosses the Interstate 78 Toll Bridge and enters New Jersey.

New Jersey

After the Interstate 78 Toll Bridge, I-78 enters New Jersey as the Phillipsburg–Newark Expressway. The road begins by running parallel with County Route 642 (CR 642) in the town of Alpha. At 3.94 miles (6.34 km), Exit 3, a partial cloverleaf interchange brings together US 22, NJ 122, and NJ 173 with Interstate 78 in Phillipsburg. 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 NJ 173. This one is located in Bloomsbury as NJ 173 begins to parallel the interstate. 4 miles (6.4 km) later, Exit 11 leaves to the right as another exit for NJ 173. CR 614 also is located off the exit. Exit 12, westbound is for NJ 173 again. However, Exit 12 eastbound is for a frontage road parallelling I-78.

Exit 13 is only westbound and is another exit for NJ 173. Nearby the exit, going eastbound, the frontage road merges in. Exit 15 is for NJ 173 and CR 513 in Franklin Township. Exit 17 is for NJ 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. 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 NJ 24 in Springfield. Exit 49A is for one of Route 24's spur routes, NJ 124. Exit 52 is for the Garden State Parkway in Union. At Exits 57 and 58, NJ 21, US 1, US 9, and US 22 intersect I-78. The exit provides access to Newark Liberty International Airport.

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. The first exit, 14A, is for NJ 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. NJ 139 runs concurrently with I-78 as it approaches the Holland Tunnel and enters New York.

New York City

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.

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 Airport. A section of I-78 at the airport was built as the Nassau Expressway, later I-878 and now 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).

Junction list

Pennsylvania

I‑81 west-northwest of Jonestown US 22 east-northeast of Fredericksburg. The highways travel concurrently to east-northeast of Fogelsville. US 222 west of Allentown

New Jersey

US 22 east-northeast of Alpha. The highways travel concurrently to Annandale. I‑287 in Bedminster G.S. Parkway in Union Township US 1 / US 9 / US 22 in Newark I-95 / N.J. Turnpike in Newark

New York

Canal Street in Lower Manhattan

Auxiliary routes

All of I-78's auxiliary routes serve New York City; however, none of these routes actually intersect I-78, following the route's truncation at the eastern end of the Holland Tunnel.

I-278 runs from U.S. Route 1/9 near Elizabeth, New Jersey over the Goethals Bridge, through Staten Island, over the Verrazano Narrows Bridge and through Brooklyn and Queens, and across the Triborough Bridge into the Bronx to end at I-95 at the Bruckner Interchange. I-278 was planned to extend west from Elizabeth to I-78 in Springfield, Union County, New Jersey, and was to intersect I-78 at the east end of the Williamsburg Bridge in Brooklyn. Until 1972, I-278 ran along I-895 and a proposed extension to I-95, while I-278 east of I-895 was I-878.

I-478 is an unsigned designation for the Brooklyn-Battery Tunnel, a spur from I-278 into lower Manhattan. Plans were made to continue it north along the West Side Highway (NY 9A) to I-78 at the Holland Tunnel, but have been canceled. Older plans would have given it the same purpose—connecting I-78 with I-278—but along the Manhattan Bridge.

I-678 runs from I-278 at the Bruckner Interchange south over the Bronx-Whitestone Bridge to John F. Kennedy International Airport. It was to intersect I-78 at its south end. Original plans took I-678 west on the Grand Central Parkway to I-278.

I-878 is an unsigned designation for part of NY 878, a short west–east freeway on the north edge of Kennedy Airport. It was once planned as part of I-78, and now intersects I-678. The number was assigned in 1989.

A former I-878 existed from 1959 to 1972 along present I-278 east of I-895. (I-895 was part of I-278.)

In eastern Pennsylvania, PA 378 into downtown Bethlehem was once I-378, but was redesignated as a state route after I-78 was rerouted to a new southerly alignment. An I-178 was initially planned as an extension into downtown Allentown, but was canceled due to local opposition.