It enters the state from Missouri near East St. Louis, Illinois, and runs to U.S. Route 41 (US 41, Lake Shore Drive) near Downtown Chicago, where the highway ends, a distance of 294.38 miles (473.76 km).

The section from the south suburbs to the area near Pontiac is officially named the Barack Obama Presidential Expressway after the president, who launched his political career from Illinois.

About eight miles (13 km) further north, I-55 intersects IL 16 at a larger interchange to the west of Litchfield.

It crosses two local roads, providing access to Elkhart and Broadwell before heading north into Lincoln.

Once entering Bloomington, I-55 forms a complex interchange with the southern terminus of the final I-55 Bus.

Continuing north, I-55 intersects local roads in industrial areas before entering Channahon.

North of I-90/I-94, I-55 intersects various city streets at partial interchanges before reaching its national northern terminus at US 41 (Lake Shore Drive).

Through the 1950s, US 66 was continually widened, straightened, and improved to handle its growing traffic, until its entire length was four lanes wide by 1957.

[8] Although the act provided for a highway replacing US 66, it was spared destruction for a while because of it being more modern than other routes at the time.

However, during the 1970s, US 66 was finally replaced by I-55 as the fourth St. Louis–Chicago highway, serving most of the same communities along the way as the original Pontiac Trail.

The earliest stretch of I-55 was a portion of US 66 which had already been built as a freeway between Gardner and I-294 (Tri-State Tollway) in Indian Head Park, and which was added to the Interstate System by simply erecting new signs in 1960.

It was renamed after Adlai Stevenson II,[11] the former governor of Illinois, on September 1, 1965, a month and a half after his death.

The Stevenson's original termini were US 66 in DuPage County to the west and the Dan Ryan Expressway to the east.

In 1999–2000, the expressway was completely rebuilt from Central Avenue north to Lake Shore Drive, including the ramps to the Dan Ryan.

The Illinois Department of Transportation (IDOT) was criticized at the time for not adding a fourth lane in each direction to the highway.

[12] In 2017, the Illinois General Assembly voted to rename approximately 70 miles (110 km) of I-55 from the Tri-State Tollway (I-294) to Pontiac in honor of Barack Obama.

[13][14] Because of the heavy traffic on I-55, IDOT spends millions of dollars per year maintaining the roadway, adding lanes, and replacing bridges to increase the capacity of the highway.

In November 2011, IDOT started a two-year pilot project, allowing Pace bus routes 755 and 855 to ride on the dedicated shoulder lanes in a given time.

[18] In the latter half of 2014, then-Governor Pat Quinn signed a bill which made the I-55 bus on shoulder service permanent.

[25] In early 2016, Governor Bruce Rauner, as well as IDOT, made a proposal to explore expanding a portion of I-55 (from I-355 to I-90/I-94) by adding an additional lane in each direction to ease congestion.