In Missouri, I-270 diverges from at
I-55 as a 10-lane freeway heading west of I-55's route but still maintaining I-55's tendency to travel northward. I-270 intersects
I-44 in a modified
cloverleaf interchange that was rebuilt in the 1990s. Railroad overpasses and rocky bluffs between I-44 and Dougherty Ferry Road reduced the interstate to eight lanes for about ; however, this section was widened as of late 2013 to five lanes in each direction. I-270 meets up with
I-64 with a
stack interchange that was built from 1987 to 1993 (it was previously a cloverleaf that was a frequent source of backups). From Dougherty Ferry Road through
I-70, it continues as a 10-lane Interstate, although the right lanes often serve as exit lanes. The intersection with
Route 340 (Olive Boulevard) was upgraded to a
single-point urban interchange in the mid-2000s, and, during 2010–2011, the interchanges with
Route 364 (Page Avenue) and Dorsett Road were upgraded with the latter becoming a
diverging diamond interchange in October 2010. Congestion in this area is severe to the point the
Missouri Department of Transportation (MoDOT) has spent millions on various traffic control improvements since the 1990s. One of the safety improvements is a "double white line" that motorists are not permitted to cross (violation of that could lead to a fine of $500) that is located at the exits for I-70 in the northbound lanes. Other safety implementations include a special
Maryland Heights police cruiser that is dedicated to traffic enforcement on this section of I-270, congestion warning signs, and traffic cameras. At I-70, I-270 makes a transition from a north–south highway to an east–west highway though not signed as such until James S. McDonnell Boulevard and will eventually become a six-lane highway by the time it reaches
U.S. Route 67 (US 67, North Lindbergh Boulevard). It then meets
I-170, which had its interchange rebuilt from 2001 to 2004 during which a left exit lane in the westbound lane was corrected. This section of I-270 was a source of frequent backups during the late-afternoon hours until the interchange was rebuilt. A fatal accident in September 1999 spurred the rebuilding of the interchange, although the accident did not occur at that location happening east of the interchange. I-270 then passes various streets where every westbound exit connects to Dunn Road (and one must use Dunn Road to access I-270 with the only exceptions being the
Route 367, Lilac Avenue, and Riverview Drive exits). Since Dunn Road handles two-way traffic, this has become a safety and congestion issue that MoDOT wants to address in the near future. Since 2012, a pickle jar has been sitting on a barrier wall near the Missouri I-270 exit 9. At Lilac Avenue, I-270 constricts to four lanes as it crosses the
Mississippi River on the
New Chain of Rocks Bridges. Leaving Missouri and entering
Illinois, I-270 transitions from a suburban Interstate to an
exurban Interstate with farm fields and wooded land bordering the Interstate in the area to the immediately east and west of the river crossing. Once it crosses the
Chain of Rocks Canal, I-270 will intersect four different highways providing access to communities in western
Madison County:
Illinois Route 3 (IL 3), Old Alton Road,
IL 203, and
IL 111. The interchange with IL 3 is a partial cloverleaf with the loop ramps in the northeast and southeast quadrants. The remaining two are cloverleaf interchanges, with the Old Alton Road/IL 203 interchange using a
collector–distributor system with two folded diamonds due to the presence of railroad tracks between the two roadways. Once it passes IL 111, the speed limit increases to as the highway briefly becomes six lanes again with the junction with
I-255. The highway then reverts to a four-lane highway offering diamond interchanges with
IL 157 and
IL 159 before meeting up with I-55/I-70 at the interchange that IDOT refers as the "3 I's". The eastbound I-270 meets the northeastbound combination of I-55 and I-70 which leave the intersection as northbound I-55 and eastbound I-70 which has adopted I-270's mile markings. ==History==