Signed on its current alignment in 1936.

In 1954, the eastern terminus was moved to Baltimore Pike via Cobbs Creek Parkway.  The route used Chestnut Street and Sixth Street for eastbound traffic and Fifth Street and Walnut Street for westbound.  The terminus was US 30 at the foot of the Ben Franklin Bridge in Franklin Park.  A median was installed in 1958 between Newtown Square and Upper Darby.

In 1962, the highway was widened and a median installed on it between West Chester and Newtown Square.  Also, the route was moved from ending at US 13 at 52nd Street via Cobbs Creek Parkway and Baltimore Avenue to 43rd Street.

In 1974, the route's eastern terminus was moved from 43rd Street to its current location.

In 1928, not only was the route's eastern terminus moved to Philadelphia via the US 22, PA 653, US 322, PA 34, PA 274, US 322, and PA 3 alignments but it was under construction from Covallen to Cove and completed in 1929.  In 1929, the route was under construction from Dunncannon to Covallen and Downingtown to Alton.  Those two sections were completed the following year.

In 1932, the route was moved onto the current PA 99 alignment from Erie to Cambridge Springs.  In 1936, the route was removed from traversing Pennsylvania from northwest to the southeast and signed in Erie County in 1936 on its current alignment except in Erie where it followed the current Alternate PA 5 alignment.  Also that year, the section from US 20 to the Fairview Township line was paved.

In 1955, the route swapped alignments with Alternate PA 5 in Erie.  Work to widen and install a median from Barnes Road to Orchard Beach Road in Northeast took place in 1958.

Median installation continued in the City of Erie in 1966 from Raspberry Street to Peach Street and Ash Street to Payne Avenue.  The last segment to be reconfigured was in 1982 at the Interstate 79 interchange, where the route was widened and a median installed from the City of Erie line to Greengarden Road as part of the interchange construction.

In 1931, the route was under construction between Mayport and the Jefferson County line and was completed the following year.  In 1934, the section between the Allegheny County line and Three Degree Road was under construction, and opened in 1935.

In 1958, construction began on the section from Grant Avenue to PA 28 in Etna.  This section opened in 1959.  The year 1958 also saw the route widened and a median installed on its from Franklin to Reno.  In 1961, the section from the end of the Richard C. Frame Memorial Highway to Franklin was upgraded with a median, and in 1968 the section from Reno to Oil City received the same.

In 1973, construction began on the section of expressway from Pearl to the northern of the expressway south of Franklin in Venango County.  That same year, its southern terminus was moved from West Carson Street in Pittsburgh to PA 28 in Etna.  In 1974, construction began from the southern end at Wesley to PA 308 and finished in 1976.  In 1977, the southern terminus was moved to its current location from PA 28 in Etna.  In 1979, the route was moved to bypass Oil City and replaced Bypass PA 8.

Signed in 1956.  In 1963, the northern terminus was moved from PA 23 in Morgantown to its current location.

Revived in 1928 to mark a route from Center Valley to Bartonsville following the PA 378, PA 512, and parrelling the current PA 33 expressway from Stockertown to Bartonsville.  This was decommissioned in 1961.

Construction on the expressway that would become PA 12 began from US 222/US 422 to Business US 222 in 1964 and finished in 1965.  In 1977, construction began on the remaining section from Business US 222 to Spring Valley Road and finished the following year.  The expressway would not receive the designation until 1999.

SR 0013 is currently assigned to US 13.

Signed in 1928 on the current alignment.  In Williamsport, it followed Washington Boulevard, Market Street and Hepburn Street for northbound traffic, while southbound used Pine Street and Sixth Street.  In 1929, the route was under construction from PA 325 to Herdon and Trout Run to Marsh Hill and both completed the following year.

In 1930, the route was paved from Millersburg to the Northumberland County line, Dalmatia to PA 325, Boile Run Road, and Dewart to Montgomery.  In 1931, the route was under construction from Fisher Ferry to Sunbury and from Ralston to Penbryn and was finished the following year.  In 1932, the route was paved from Millersburg to Dalmatia.  In 1936, the southern terminus was moved from Millersburg to Harrisburg when the route was moved from the eastern shore to the western shore to multiplex with US 11 on that side.  

In 1941, it was moved from the western shore of the Susquehanna from Wormleysburg to Northumberland to the eastern shore.  From Milton it followed the current PA 405 alignment to Muncy.

In 1955, the southern terminus was moved from Oakleigh at Paxton Street to Clarks Ferry.  In Harrisburg it followed South 40th Street, Derry Street, Mulberry Street, and Front Street.

In 1963, the southern terminus was moved from US 22/US 322 in Clarks Ferry to its current location.  Prior to the move it came in via Washington Boulevard and High Street.

From 1928 to 1930, it was on the current PA 317 alignment from the Ohio state line to Mt. Jackson.  From 1930 to 1933, it was signed on the current US 224 alignment.  In 1930, the section from the Ohio state line to Peanut was paved.

The current alignment received the PA 17 designation in 1928, but only from Millerstown to Liverpool.  In 1936, the western terminus was moved from Millerstown to its current location.

From 1987 to 1999, there were two PA 17s, with the other one designating the current I-86 to continue the NY 17 designation.

In 1928, the route was moved to its current alignment between PA 158 and Greenville.  Originally, it traveled the current PA 158 and PA 58 alignments.  That year it was under construction from US 30 to Service Church Road and Hill Road to PA 198 and completed the following year.  In 1929, the route was under construction from Harshaville to Service, which was finished the following year.  Also that year, paving was done from Nettle Hill to Whitecottage, Gretna to current PA 50 and Shaffer Road to Shippingport.

In 1932, the section from West Virginia to Nettle Hill, Florence to US 30, and Green Garden Road to Mowry Road was paved.  In 1935, the section from Nineveh to the Washington County line was paved.  A year later, the section from the Greene County line to Old Concord was paved.

In 1958, the route was widened and a median installed between PA 551 and Koppel.

In 1963, the designation was moved from Liberty Street, Atlantic Avenue, and Washington Street in New Castle to its current alignment on Mahoning Avenue and Jefferson Street.  In 1967, the route was widened and a median installed from PA 60 to Monaca.

In 1970, the route was widened and a median installed from PA 518 to Clark.  Also that year, work to widen and install a median on the route from Clark to Transfer began. Construction was extended from Transfer to Shenango in 1971 and in 1972, work on those sections were completed.  In 1971, the route was widened and a median installed from US 62 to Highland Road.

The designation was moved to the PA 60 expressway from its current alignment between the West Middlesex interchange and the end of the Beaver Valley Expressway in 1970.  Moved back in 1978 and eliminating the Business PA 18 designation through West Middlesex.

In October 2001, work began to widen the highway to five-lanes in Hermitage City.  This project finished in November 2002.

For years, Geneva College in Beaver Falls has lobbied PennDOT to rectify a dangerous S-curve near the campus.  Aside from safety, the route cut into part of the campus which prevented expansion.  In Fall 2006, PennDOT began work to realign PA 18 which was completed in November 2007.

In 1932, the route was paved from Coal Lick Road to Baileys Crossroads, Crucible Road to Jacobs Ferry, and from East Riverside to Hibbs.

In 1946, the route was changed to follow the current alignment between Paisley and Uniontown.  Prior to that, it went northeast from Carmichaels via Arensburg Road, Penncraft Road, Millsboro Road, New Salem Road, Herbert Road, and Dearth Road and then multiplexed with US 40 to end at PA 51 at Pittsburgh Street in Uniontown.

In 1953, the route was shifted from Georges Road between Baileys Crossroads and Carmichaels to its current route between Baileys Crossroads and Paisley.  Also that year, a new alignment was under construction from west of Masontown to east of McCellandtown which opened in 1954.  The highway was widened and a median installed from east of Paisley, Masontown and Leckone, and in McClellandtown in 1958.

Morrisville to Curry Home Road was opened in 1966 moving the designation off Rolling Meadows Road and Curry Home Road.  A median was installed from east of Paisley to the Monongahela River in 1967.

With PennDOT restructuring their 12-Year Transportation Program, the project to improve PA 21 in Greene and Fayette Counties will be reevaluated.

In 1957, the route was moved from North Spring Road and Conshohocken State Road to Gulph Road, Springs Mill Road, Old Gulph Road, Morris Avenue, and Mill Creek Road between Valley Forge and Philadelphia.

In 1965, the western terminus was moved from Lancaster to its current location.  In 1967, it was moved from Gulph Road, Springs Mill Road, Old Gulph Road, Morris Avenue, Mill Creek Road to its current route between Valley Forge and Philadelphia.

Construction began in 1976 on the "goat path expressway."  In 1978, the expressway project was cancelled by locals not wanting all the excessive traffic.  The "goat path expressway," as it is referred to was covered by dirt.  However, there has been talk of reviving this project to accommodate growing traffic levels.  PennDOT has been studying how to improve PA 23 from US 30 to PA 772, which is where the original expressway would have been built.  However, now that the restructured 12-Year Transportation Program has been released, this is now being reevaluated.

In 1984, the eastern terminus was moved from the intersection of US 30/Girard Avenue and Belmont Avenue to the current location.  Construction began on the alignment from Walnut Street in Lancaster to US 30 and opened in 1993.  This caused the route to be moved off New Holland Pike from Walnut Street to US 30 and onto this new alignment.