In the U.S. state of Colorado , Interstate 25 follows the north–south corridor through Colorado Springs and Denver . The highway enters the state from the north near Carr and exits the state near Starkville . The highway also runs through the cities of Fort Collins , Loveland , and Pueblo . The route is concurrent with U.S. Highway 87 through the entire length of the state. I-25 replaced U.S. Highway 87 and most of U.S. Highway 85 for through traffic.
Historical nicknames for this route have included the Valley Highway (through Denver ), Monument Valley Highway (through Colorado Springs ), and the Pueblo Freeway (through Pueblo). Within El Paso County , the route has been dedicated as the Ronald Reagan Highway .   In Pueblo County , the route is called John F. Kennedy Memorial Highway .
New Mexico state line to Pueblo
Following the Santa Fe Trail from New Mexico , Interstate 25 enters Colorado as a typical four-lane Interstate Highway, where its entire route in Colorado lies close to the east side of the Rocky Mountains . The route turns from north to west-northwest as I-25 serves Wootton. After leaving Wootton, I-25 turns back up north and bypasses near the east side of the Trinidad Lake State Park , home of the Trinidad Lake .
Trinidad , a city near the Trinidad Lake, is the first major city that lies along I-25. For the next 30 miles (48 km), I-25 continues north through the rural areas of Colorado until it reaches the small city of Walsenburg , where the business route - I-25 Bus. - junctions with U.S. Highway 160 . I-25 then continues in a north-northwest direction until it bypasses the Orlando Reservoir, then turns north from there until it reaches Colorado City . In Colorado City, I-25 interchanges with the east end of the Frontier Pathways Scenic and Historic Byway ( SH 165 ) at exit 74.
After leaving the city, I-25 follows in a north-northeast orientation until it reaches the St. Charles Reservoir just before entering the city of Pueblo , with the first exit within the southern city limits of Pueblo at exit 94. The Arkansas River in Pueblo serves as a feeder to the Lake Pueblo State Park , home of the Pueblo Lake, which is located to the west of the western city limits of Pueblo. 
Pueblo to Denver
After leaving Pueblo , I-25 continues up north with the Union Pacific Railroad line paralleling closely to the route on the right side after interchanging with Porter Draw at exit 106. By exit 119, the Fountain Creek joins along and travels parallel with I-25, and continues all the way to the Fountain Creek Regional Park in Widefield . I-25 gradually turns from a general north direction to the north-northwest and serves the census-designated place of Buttes at exit 122.
As soon as US 85 leaves I-25 at exit 128, I-25 enters the city limits of Fountain . Basically, I-25 serves as the border between the western city limits of Fountain on the east side of I-25 and Fort Carson on the west side. Exit 132 ( SH 16 ) serves the north side of the Fountain Creek Regional Park as well as the entrance to Fort Carson and connects to SH 21 (Powers Boulevard), the eastern bypass for the Colorado Springs metro area . By the time I-25 reaches exit 138, the route crosses into the city limits of Colorado Springs , where the stack interchange with US 24 at exit 139 serves the Evergreen Cemetery and Prospect Lake. I-25 turns west at exit 140, along with the Fountain Creek , where it interchanges with US 85 , US 87 , and I-25 Bus. I-25 again turns back north by exit 141. Swinging around the west side of downtown Colorado Springs at exit 142, and to the north of the city lies the Colorado College , and is served at exit 143 - Uintah Street. Continuing north and northeast, the highway intersects the north terminus of I-25 Bus. and US 85. The interstate leaves Colorado Springs between exits 153 and 156, where I-25 enters the United States Air Force Academy , going through the east side of the institution.
I-25 leaves El Paso County and enters Douglas County at Monument Hill , elevation 7,352 feet, north of Monument . I-25 then continues north through more rural and hilly areas east of the Rocky Mountains until reaching Castle Rock at exit 181. I-25 continues through rural and hilly portions of Douglas County until interchanging with E-470 , the partial beltway of Denver as the toll road serves the Centennial Airport and the much larger Denver International Airport .
After entering Arapahoe County , I-25 cuts through the Denver Technological Center (DTC) between Dry Creek Road and Belleview Avenue (exits 196-199). I-25 enters Denver at the I-225 interchange, a spur that detours motorists to I-70 through Aurora , at exit 200. I-25 turns in an westerly direction between Evans Avenue (Exit 203) and Colorado Boulevard (Exit 204). University of Denver lies just to the south of the interstate at Exit 205. It then turns back north after Exit 207. I-25 curves around the west side of downtown Denver , where it can be accessed by I-70 Bus. at exit 210. I-25 then interchanges with I-70 at exit 214 right before leaving the City and County of Denver . 
Denver to Wyoming state line
As I-25 leaves Denver, the route continues up north through unincorporated areas of Adams County and interchanges with I-76 , I-270 , and the Denver-Boulder Turnpike ( US 36 ). Due to the complexity of this triangle-shaped interchange, it was known to be one of many malfunction junctions throughout the United States . Beyond that interchange, the interstate enters the northern suburbs of the Denver metro area, such as Thornton and Northglenn , and at exit 220, I-25 slips its way through a narrow path between the Badding Reservoir (west side) and the Croke Lake (east side). Development begins to drop off after exit 223 (120th Avenue) after continuing north into Westminster and eastern Broomfield .
At exit 228, I-25 interchanges with the northern termini of E-470 and Northwest Parkway at a stack interchange , with the Larkridge Mall just to the north, served by 160th Avenue ( SH 7 ). As I-25 continues north, it moves through rolling farm and grasslands with the Front Range and high mountains clearly visible to the west while passing through a medley of lakes and reservoirs. It stays generally flat with few moderate climbs in elevation, while also serving smaller cities like Dacono and Firestone to the east and Longmont to the west. This stretch of I-25 in northern Colorado also has large amounts of truck traffic between SH 7 and Wyoming . After some time in the rural farmlands, the interstate enters the Fort Collins/Loveland metro area at exit 255, serving Loveland and Greeley to the east at exits 255 and 257, and continuing north to the Fort Collins city limits south of Harmony Road. The highway runs on the eastern side of Fort Collins, serving Colorado State University at exits 268 and 269 (which is also the most direct route to downtown). After exit 271, I-25 leaves Fort Collins and rolls into more rural grasslands past Wellington . Exits also become few and far between from here to Wyoming as well after gradually turning north-easterly towards the state line. 
Ancestors and early freeways
State Highway 1 , an unpaved road, was completed between Denver and Pueblo by 1919. Average travel time between Pueblo and Colorado Springs on this route was approximately 2.5 hours (or a full 8.5 hours from Pueblo to Denver). This route was upgraded with the help of the federal government to become US 85 and US 87 by 1930, now paved in concrete and shortening the travel time between Pueblo and Colorado Springs to just one hour.
The cities of Denver (in 1948) and Pueblo (in 1949) were first to begin building multi-lane highway segments along the route of what would eventually become Interstate 25. Construction follows an earlier segment of the Colorado and Southern Railway . Denver's segment was originally known as the Valley Highway and was completed by 1958. The city of Colorado Springs followed a similar theme with their Monument Valley Freeway , begun in 1955 and completed by July 1960. Pueblo's section — the Pueblo Freeway - was complete by July 1959. 
As the national Interstate Highway System began to take shape, actual "inter-state" connections began to be made. Wyoming came first in 1964, building a 9-mile (14 km) link north to Cheyenne that was connected to Colorado 's 17-mile (27 km) stretch.
Linking to New Mexico in the south would prove more problematic as the planned route had to stretch over Raton Pass , and its accompanying 1,800-foot (550 m) elevation change, within just 13 miles (21 km). Once again, US 85 and US 87 were used, but it had to be re-graded in places to meet Interstate design guidelines. Construction began in 1960, with a link to the city of Trinidad completed by 1963. The Trinidad Segment (as CDOT now calls the Raton Pass span) was not fully completed until 1968.
The final segment of the Colorado portion of Interstate 25, connecting the cities of Walsenburg and Trinidad , was completed during 1969. This meant that four lanes of high-speed, nonstop freeway were finally open for a full 305 miles (491 km) from New Mexico north to Wyoming . 
As both population and traffic increased in Colorado during the 1990s and 2000s, the Colorado Department of Transportation has planned and completed major improvements for the city corridors along I-25.
The first of these was Transportation Expansion ( T-REX ), which widened and expanded nearly 17 miles (27 km) of both I-25 and the I-225 bypass in the Denver Metropolitan Area as well as adding various pedestrian and aesthetic improvements. T-REX was also instrumental in expanding Denver's RTD light rail lines to connect outlying communities beyond the city and county of Denver , adding 19 miles (31 km) of new routes. 
Starting in early 2004, the T-REX project was completed during 2006 at a cost of US$1.67 billion, under its projected budget and two years ahead of its originally scheduled conclusion. It has been hailed as a "model for other cities to follow" and "ahead of the curve nationally" by federal transportation and transit authorities. 
COSMIX (Colorado Springs)
As T-REX began to wrap up, CDOT's next major effort began with Colorado Springs Metro Interstate Expansion ( COSMIX ). It could be argued that COSMIX was even more important to Colorado's interests than T-REX had been, since the Colorado Springs corridor of I-25 had seen immense growth over the past four decades, and experienced major choke points all along the 16-mile corridor from Exit 135 (Academy Blvd) in the south to Exit 151 (Briargate Pkwy) in the north. Originally carrying around 8500 vehicles per day in 1960, usage of the former Monument Valley Freeway had grown to an average of 100,000 vehicles per day by 2005.
The major goals of COSMIX, which began in 2005 and was completed a year and four days ahead of schedule at the very end of December 2007, were a general expansion and widening of the corridor to three lanes in each direction throughout the city, as well as the reconstruction of two main interchanges (at Bijou Street near downtown Colorado Springs , and at Rockrimmon Boulevard and North Nevada Avenue in the city's growing north side).  Originally estimated at $225 million, on delivery COSMIX cost only $150 million, approximately $20 million of which involved land acquisition costs. With respect to schedule and budget, then, COSMIX proved a superbly successful project, the result of the decision by the project manager and project engineer, both young women, to negotiate with CDOT for six months' extra detail planning time up front. Their efforts paid off in an effective reduction of the overall work schedule by 18 months and 4 days.
COSMIX was the first funded portion of a larger plan for I-25 improvements as detailed in an Environmental Assessment approved by CDOT and FHWA in 2004. A second phase resulted in the widening of the 12 mile segment from Woodmen Road (exit 149) to Monument (exit 161) to six lanes and addition of auxiliary lanes at busy interchanges. The Air Force Academy interchange (exit 156) was reconfigured to include just one exit, instead of A/B, and features two new roundabouts for North Gate Boulevard. The widening and paving was completed in December 2014. 
An EA-recommended improvement not included in COSMIX due to funding limitations was the reconstruction of the I-25 interchange at Cimarron Street (US 24 West). CDOT completed this project in late 2017.
A new "lane balance" project has begun in northern Douglas County (Lone Tree) from Lincoln Avenue to County Line Road, where CDOT will expand the highway to eight lanes to eliminate the current six lane bottleneck from drivers coming from Castle Rock to the south into Denver. This will connect the existing eight lanes from RidgeGate Parkway to the terminus of the previous T-REX project from years earlier at County Line Road. 
Toll lanes will be added along I-25 from US 36 (exit 217) to 120th Avenue (exit 223) in the Thornton area starting in October 2013. Two lanes (one north, one south) will be operational for drivers wishing to cut through traffic by paying a toll, similar to the US 36 project. The highway will have to be expanded and reconstructed to add the additional lanes to the six lane configuration currently in place. The project should be complete by October 2015. 
There is much controversy surrounding the future of Interstate 25 in northern Colorado (SH 7 in Broomfield to SH 14 in Fort Collins). Suggestions from adding toll lanes to general expansion to six lanes from the two lane bottleneck at SH 66 to SH 14 and adding multi-modal transportation options have been discussed. The future of the highway remains in question as funding is limited, and agreement is limited as well. The I-25 corridor in Weld and Larimer counties is becoming increasingly heavy with traffic, and something will have to be done soon. 
The seven mile segment of I-25 through Pueblo also is in need of reconstruction, and the subject is of much controversy in southern Colorado. On the agenda are expanding the now 60-year-old interstate to six lanes, adding additional auxiliary lanes, and improving many interchanges to bring them up to current standards. The road segment from exit 101 (US 50/SH 47) to exit 94 (SH 45/Pueblo Boulevard) is currently the oldest section of I-25 in the state, and has not been upgraded since the 1950s, minus minor alterations and paving. The controversy surrounds the straightening of the highway through the city and the possible removal of several businesses around the downtown area and the historic district.