Over two thirds of PA 51 is 4 laned. (Including the entire route from US 119 in Uniontown to the West End Circle in Pittsburgh.) It is considered the main route from Pittsburgh to the Mon Valley and Fayette County. And sadly, PA 51 is under construction frequently.  From US 119 to Elizabeth, it is very much similar to a rural at-grade four lane divided expressway.  It then turns suburban through the eastern most reaches of the South Hills where it is the main access road to Century III Mall.  Upon crossing into the city limits of Pittsburgh, Route 51 becomes part of Saw Mill Run Blvd. where gridlock and aggravation prevails.  It then visits McKees Rocks and Coraopolis as the highway parallels the shores of the Ohio River until crossing it near Beaver.  The highway returns to its rural four lane roots in western Beaver County into Ohio.

History:
Uniontown to Monongahela River Crossing (Elizabeth):
For over 30 years, PA 51 snaked its way through the Mon Valley towards Uniontown.  Known mostly as 'Pittsburgh Road,' route 51 was a tight two lane road with various hills and oxbows. Although predominantly rural, PA 51 did go through the downtown's of Perryopolis and Elizabeth.  By the late 1940's, 51 moved out of central Perryopolis on a 'bypass' about a mile to the west.  Elizabeth was bypassed in 1951 with the completion of the Elizabeth Bridge.  Previously 51, entered Elizabeth via Market St., turned left onto Second Avenue and right onto Plum St. before crossing the Monongahela into West Elizabeth.  During the 1950's PA 51 south of Elizabeth was straightened and widened to four lanes.  By 1953 (Image #1), 51 was widened from Elizabeth to PA 48, and from the Westmoreland/Fayette County Line south to PA 711 (now PA 201).  By 1957 most if not all of PA 51 was widened in Westmoreland County, this includes the Rostraver Bypass that stretches from PA 201 south to I-70.  The 1960's saw the completion of the widening project to Uniontown.  It wasn't until 1996 that the last section of PA 51 south of Pittsburgh was widened to four lanes.  The stretch of 51 from Perryopolis to Star Junction was widened from two lanes with center turn lane to four lanes with center turn lane.

Monongahela River Crossing to West End Circle:
The pre-1930 routing of Route 51 saw the road enter into West Elizabeth from the old Elizabeth Bridge, enter Large via Scotia Hollow and McGrew Hollow Roads.  51 then was carried above the Lewis Run Valley along Clairton Road through Jefferson Township (now Jefferson Hills and Pleasant Hills Boroughs).  It then similar to the route south of the Monongahela, climbed numerous hills until descending into the Saw Mill Run Valley and into the West End.  In 1930, Saw Mill Run Blvd. opened from Library Road to Banksville Road.  The new roadway eliminated many oxbows and provided a four lane roadway linking the South Hills to the Liberty Tunnels.  Later in the 30's, 51 was moved into the Lewis Run Valley along a three lane road from Large to Lebanon Church Road.  This would include Allegheny County's first Cloverleaf Interchange opening in 1939 at Lebanon Church Road.

Through the 1940's, 51 was straighten and widened between the Cloverleaf and Library Road; and by 1951, two more towns, West Elizabeth and the West End were bypassed.  With a proposal to connect Large and the new Elizabeth Bridge via tunnel failing, West Elizabeth was bypassed by 51 ascending the Ridge and funneling into Large.  The West End Bypass (See Image #2), originally proposed by Robert Moses, was built from the Banksville Traffic Circle to the West End Bridge.  The bypass eliminated the well known bottleneck in the area.  Improvements were made to the Lewis Run section, as it was widened from three lanes, which had a very dangerous suicide passing lane, to four in the late 50s.  The Banksville Circle was eliminated with the construction of the Parkway West in the late 50s.

The 60's & 70s saw a mix of expressway and widening plans for Route 51, specifically along Saw Mill Run.  However, numerous local opposition groups blocked many proposals.  Since then, 51 has mainly seen numerous, if not to those who use it for their commute, endless rehabilitations and minor upgrades.  The most significant was the Split-Diamond Interchange at the South Portal of the Liberty Tubes.  Also, Route 51 was widened and straighten yet again in the Lewis Run Valley to accommodate an extra turning lane.  This work was done by the PA Turnpike Commission in conjunction with the PA 43 freeway.

West End Circle to Ohio River Crossing (Beaver):
Most of the changes did not occur until the 1960s.  PA 51 was widened from Coraopolis to Broadhead Road in the 1960's.  Also, a new and four laned Broadhead Road was built in the Monaca Area.  In 1976, PA 51 was moved from Narrows Run and Broadhead Roads onto the former alignment of PA 930 paralleling the Ohio River.  (See Image # 3)

Ohio River Crossing to Ohio State Line:
Not much has changed to Route 51 in this area except for changes in the Rochester Area done in the 1960s. Most of the work included new four lane highways and grade separated intersections.

Bannered Routes:

Alternate PA 51: Uniontown
Not sure on the years or the signing of this route.  However, it is shown on the Uniontown Inset of the 1953 Official.  (See Image #4). It left PA 51 at Mt. Vernon St. then turned East on US 40 (Fayette St.) and ended at US 40, US 119 and PA 51.

Truck PA 51 South: Elizabeth
Signed to bypass trucks from Elizabeth-McKeesport Road heading to PA 51 South from Downtown Elizabeth.  Begins at the Second Ave. Ramp to PA 51 North, crosses over the bridge, and ends at the U-turn ramp to PA 51 South.  (See Image #5).

Finding the Old Road:
Has there own separate pages:

#3: 1940 Official.  PA 51 was moved onto the former
alignment of PA 930 in 1976.
 

#4: 1953 Official. Alternate PA 51
 

#5: Truck South PA 51, Elizabeth.  (William Lawson)