PA 309 then enters the Lehigh Valley, where it joins Interstate 78 (I-78) on a freeway bypassing Allentown to the south before splitting to the north and running through rural areas as a surface road.

[3][4][5] At this point, PA 309 turns north-northwest onto four-lane divided Ogontz Avenue and fully enters Cheltenham Township in Montgomery County, passing businesses as it heads to the west of the shopping center.

From there, the route heads northwest and passes to the southwest of Arcadia University and to the northeast of Holy Sepulchre Cemetery before it runs near wooded areas and suburban residential development, crossing into Springfield Township.

The freeway curves west and comes to a diamond interchange at Paper Mill Road, where it makes a turn to the northwest as it passes northeast of Springfield Township High School.

From here, the freeway runs near business parks before heading north-northwest through wooded residential areas to a partial interchange with Highland Avenue consisting of a northbound exit and southbound entrance.

The route heads north through more suburban development to the east of Ambler, passing west of Upper Dublin High School, and reaches a northbound exit and southbound entrance at Susquehanna Road.

The freeway runs through woodland and residential development, crossing into Lower Gwynedd Township and turning north to reach a diamond interchange serving Norristown Road to the east of the community of Spring House.

The route curves to the northwest and heads through wooded areas, splitting into a one-way pair carrying two lanes in each direction and reaching an intersection with the southern terminus of PA 378 in Center Valley.

The highway comes to a southbound exit and northbound entrance with Rock Road that provides a connection to PA 145 in Summit Lawn, at which point it crosses into Salisbury Township.

The highway runs near industrial areas and passes over Norfolk Southern's Reading Line before it comes to a partial cloverleaf interchange at Lehigh Street.

The freeway heads back into Salisbury Township and runs between residential areas to the north and office buildings to the south before reaching a partial cloverleaf interchange with the northern terminus of the southern section of PA 29 at Cedar Crest Boulevard.

Past this interchange, the highway heads north of Lehigh Valley Hospital–Cedar Crest and crosses into South Whitehall Township, passing between residential areas to the north and farm fields to the south and curving northwest to come to a partial cloverleaf interchange with the northern terminus of US 222 and the southern terminus of PA 222 at Hamilton Boulevard, which provides access to Allentown and Reading.

[3][8] Past the US 22 interchange, the freeway ends and PA 309 continues northwest as an unnamed four-lane divided highway with at-grade intersections, passing near commercial development.

PA 309 curves north and heads through a mix of farm fields, woodland, and homes and businesses, passing through Guthsville, where it crosses Jordan Creek, and Orefield.

The route runs through more rural land with occasional development and reaches Pleasant Corners, where it crosses Jordan Creek and comes to an intersection with the northern terminus of PA 100.

PA 309 continues west through agricultural areas with some woods and homes and passes south of Northwestern Lehigh High School as it enters Lynn Township.

The route becomes two-lane Center Street and runs past businesses, crossing the Little Schuylkill River and a Reading Blue Mountain and Northern Railroad line at-grade.

Past this interchange, PA 309 heads through forests with some development before passing under a Reading Blue Mountain and Northern Railroad line and entering McAdoo.

[3][9] The route enters Banks Township in Carbon County and becomes unnamed, heading to the east of a coal mine before running past homes in Audenried.

At this point, PA 309 crosses back into Hazle Township and runs through commercial areas as an unnamed road, passing to the west of Church Hill Mall and widening to five lanes.

The road runs through forests with some commercial development, passing east of Crestwood High School and crossing Bow Creek before entering Fairview Township.

The road runs through forested areas as it passes through Solomon Gap in Penobscot Mountain, narrowing to two lanes before turning into a four-lane divided highway.

The freeway bends farther from the business route and passes through Georgetown, running through wooded areas and coming to a trumpet interchange which provides access to Highland Park Boulevard, serving multiple shopping centers and the Mohegan Sun Arena at Casey Plaza, where the Wilkes-Barre/Scranton Penguins of the American Hockey League play.

The route follows the North Cross Valley Expressway through wooded areas with adjacent development, coming to a partial cloverleaf interchange with the northern terminus of PA 309 Bus.

Past here, the freeway passes over a Reading Blue Mountain and Northern Railroad line and the Susquehanna River, at which point it enters Forty Fort and comes to a northbound exit and southbound entrance at Rutter Avenue that provides indirect access to US 11.

The freeway reaches a northbound exit and southbound entrance at Union Street, where it narrows to four lanes and crosses into Pringle, heading across Toby Creek.

[3][12] The route enters Monroe Township in Wyoming County and continues through forests with some fields and homes to the west of Leonard Creek as an unnamed road, bending northwest and passing through Beaumont.

[3][13] The portion of PA 309 between Philadelphia and the Lehigh Valley follows the routing of a Native American path now referred to as the "Minsi Trail" that dates back to the 18th century.

[50] The remaining section of the North Cross Valley Expressway from Wilkes-Barre Boulevard to I-81 was completed on November 9, 1991, with Governor Robert P. Casey in attendance for a ribbon-cutting ceremony.

[58] On November 11, 2019, work began on a project to improve the section of PA 309 along the Sellersville Bypass in Bucks County by rehabilitating pavement and reconstructing structures.