Before the construction of I-25, US 85 was the through route. This road was already somewhat developed in the early 20th century, before 1930 a long stretch from Colorado Springs via Denver to Greeley was asphalted, as one of the first long paved roads in Colorado. Around 1930, the section between Pueblo and Colorado Springs was also asphalted, as well as the approach roads to the southern city of Trinidad. In the second half of the 1930s, the last stretches were paved, between the New Mexico and Trinidad border, between Trinidad and Pueblo, and between Greeley and the Wyoming border.

Later, US 87 emerged as the second north-south route through this part of Colorado. North of Denver, it ran parallel to US 85 slightly more to the west. Between US 85 and US 87, a third route was developed from the 1940s onwards, which would later be transformed into the route of I-25. US 87 was later moved to this middle route, US 287 was then routed over the westernmost north-south route north of Denver.

In the 1950s, three traffic routes were created between Denver and Colorado Springs, the original route was the westernmost route via Sedalia, which at the time was double-numbered as US 85/87. In the early 1950’s, two new roads were completed, a middle route over which US 87 and later I-25, passing through Castle Rock, was routed, and an easterly route through Franktown, which was numbered SH 83.

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Construction history

Even before the creation of the Interstate Highway system, there were plans to build a highway along the foothills of the Rocky Mountains, traditionally the most densely populated region of Colorado. In 1944 the first plans were worked out. Shortly thereafter, work began on widening existing access roads to 2×2 lanes, in Denver in 1948 and Pueblo in 1949.

The construction of these approach roads later went on to become a highway. At the end of the 1950s, the first highway sections were under construction at Denver, Colorado Springs and Pueblo. The first section opened in November 1958, this was the west bypass of downtown Denver. Shortly after, in July 1959, the Pueblo passage opened to traffic.

In the second half of the 1950s, extensive work was done to widen US 85/87 south of Denver to 2×2 lanes. Initially this was largely a 2×2 divided highway, of which only parts were grade separated. By 1960, a 150-mile stretch from Pueblo to Mead north of Denver had been widened to 2×2 lanes. Much of it was already grade separated, but not all, such as between Colorado Springs and Castle Rock and parts north of Denver and north of Pueblo. These grade-separated connections were added in the subsequent period.

The first grade-separated intersections between Trinidad and Walsenburg were also constructed in the early 1960s, as well as the missing grade-separated intersections north of Denver. A major opening followed on October 12, 1964, when the northernmost portion of I-25 opened to the Wyoming border. At that time, however, not the entire route from Denver to Cheyenne was a freeway, near Fort Collins was a longer section of single lane.

Construction of I-25 was completed in the second half of the 1960s. This happened along Fort Collins in 1968, between Colorado Springs and Denver in 1968, between Walsenburg and Pueblo in 1968 and finally the last section between Trinidad and Walsenburg in 1969.

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Widenings

The northern suburban portion of Denver was widened to 2×3 lanes as early as the 1980s or earlier. In the mid-1990s, I-25 north of downtown Denver was widened with a switch lane and new lanes. In the period 2003-2004, the I-25 between E-470 and Longmont was widened to 2×3 lanes. In 2008, the 2×3 section was extended a little further north to Mead.

Around 2002, I-25 between Castle Rock and Centennial was widened to 2×3 lanes. In the period 2014-2016, the highway in the south of Centennial was further widened to 2×4 lanes. In 2013, the widening of I-25 between Lincoln Avenue and County Line Road in the southern suburbs of Denver started. This section has been widened to 2×4 lanes. The widening was completed in March 2016.

Between 2013 and 2016, a two-way express lane was constructed on I-25 in the north Denver metropolitan area, between the interchange with US 36 and 120th Avenue, for 10 kilometers. These express lanes connected to the older interchange lane from downtown Denver. The express lanes began construction in October 2013 and opened to traffic on March 14, 2016.

The section through Colorado Springs was gradually widened to 2×3 lanes in the period 2003-2009. On November 1, 2014, the widening of 17 kilometers of I-25 between Colorado Springs and Monument was completed. The highway has been widened to 2×3 lanes here.

Between 2015 and 2018, I-25 was reconstructed by Pueblo. I-25 through Pueblo was one of the first freeway sections built in the late 1950s. All works of art have been replaced by modern viaducts with contemporary design requirements. Work started on 1 July 2015, and was completed in 2018.

Between 2019 and 2021, the last 29 kilometers between Castle Rock and Monument has been widened to 2×3 lanes. This project was called the I-25 South Gap. In October 2017, funding of $350 million was found for this. Work began in June 2019 and opened one year ahead of schedule on December 10, 2021. This widened the final section between Denver and Colorado from 2×2 to 2×3 lanes. The new lanes are express lanes with toll collection. This provided a minimum of 2×3 lanes for 174 kilometers of I-25 through central Colorado.

T-REX

In the 1990s, the rapid population growth around Denver put a lot of pressure on I-25. This resulted in T-REX (TRansportation EXpansion), the 17-mile widening of I-25, as well as aesthetic improvements and pedestrian bridges. 31 kilometers of light rail were also constructed along the highway. The project was completed in 2006 and cost $1.7 billion, well under budget, and 2 years ahead of schedule.

Opening history

From	Unpleasant	Length	Opening
Exit 203	Exit 215	19 km	11-1958
Exit 94	Exit 100	10 km	07-1959
Exit 100	Exit 161	98 km	1961
Exit 229	Exit 243	23 km	1961
exit 13	Exit 15	3 km	1963
Exit 215	Exit 229	23 km	1963
Exit 243	Exit 257	23 km	1963
Exit 49	Exit 52	5 km	1964
Exit 288	Exit 300	19 km	12-10-1964
Exit 74	Exit 94	32 km	1965
Exit 161	Exit 173	19 km	1965
Exit 257	Exit 265	13 km	1965
exit 0	exit 13	21 km	1966
Exit 52	Exit 74	35 km	1968
Exit 173	Exit 182	14 km	1968
Exit 182	Exit 203	34 km	1968
Exit 265	Exit 288	37 km	1968
Exit 15	Exit 49	55 km	1969

Future

Denver – Wyoming

It is planned to widen a 47-kilometer section of I-25 between SH 66 in Longmont and SH 14 in Fort Collins to 2×3 lanes. On January 4, 2018, a $248 million contract was signed for the widening between Johnstown and Fort Collins, with the third lane becoming an express lane. The extra lanes should be opened by mid-2023.

Toll

express lanes

Express lanes are partially present on I-25 through Denver. There is a short interchange lane between 20th Street and US 36, which is toll-free for HOV vehicles but where solo drivers pay toll. In addition, there are express lanes (one in each direction) north of it between US 36 and 120th Avenue in the north of the city. This connects to the interchange lane and express lanes from US 36 to Boulder.

South of Denver, there are also express lanes between Monument and Castle Rock, where tolls are payable. There are also express lanes further north of Denver, between Berthoud and Fort Collins. These two sets of express lanes are primarily funding models for widening those sections of I-25 and less to regulate traffic. Express lanes are not normally common as third lanes.

Traffic intensities

In southern Colorado, the intensities are low, around 10,000 vehicles per day. In Pueblo this rises to 72,000 vehicles per day. In Colorado Springs there are 105,000 mvt/day, which drops to 54,000 vehicles between Colorado Springs and Denver. In Denver, the highway is a lot busier, with 217,000 vehicles per day in Centennial. 220,000 vehicles per day drive past the center, and 258,000 vehicles per day on the I-70. This can all be settled well. North of Denver, the highway will remain busy for a long time, with 92,000 vehicles north of E-470. It’s not until Fort Collins that the highway becomes quiet again with less than 50,000 vehicles. 17,600 vehicles cross the Wyoming border every day.

Lane Configuration

From	Unpleasant	Lanes	Comments
exit 0	Exit 135	2×2
Exit 135	Exit 184	2×3
Exit 184	Exit 194	2×4
Exit 194	Exit 197	2×5	Centennial
Exit 197	Exit 200	2×6	Denver
Exit 200	Exit 212	2×4	Denver
Exit 212	Exit 217	4+2+4	alternating lane
Exit 217	Exit Exit 223	2×4	express lanes
Exit 223	Exit 243	2×3
Exit 243	Exit 299	2×2