Drive Colorado 119 past Sandstone Ranch this fall and you'll hit the same setup that's been there since March: two lanes each direction, split by concrete barrier, speed limit dropped to 45. Walk into the Sandstone Ranch parking lot near the Visitor Center and you'll find a chunk of it fenced off, has been since before Christmas. None of this is new information if you drive that stretch or walk that trail. What's changed is the finish line.
The city's own construction bulletin confirmed in August that the traffic pattern on 119 will hold through winter while crews build a pedestrian underpass beneath the highway. A March 2026 report from CBS Colorado on the same project went further, stating plainly that construction is slated to wrap by spring 2027. That's a full year past the completion window Longmont's planning documents have quoted for this project since the design contract was awarded back in January 2022, which pegged construction to run from fall 2024 to spring 2026. If you've been checking the city's general project background page and taking that spring 2026 date at face value, you're working from an estimate the ground-level construction updates have already passed by.
Two Miles, Two Bridges, One Underpass
The project is officially Phase 13 of the St. Vrain Greenway, and it's the piece that takes the trail past its current eastern edge. Right now the Greenway runs roughly eight miles, from Golden Ponds Park on the west side to Sandstone Ranch Nature Area on the east, a route that already connects to Roger's Grove, Izaak Walton Park, and the Boulder County Fairgrounds along the way. Phase 13 adds more than two miles beyond Sandstone Ranch, extending the trail northeast into the trail network at St. Vrain State Park.
Getting there means crossing Highway 119, which is the expensive part. The plan calls for two pedestrian bridges over St. Vrain Creek, one north of 119 and one south of it, plus a concrete underpass built beneath the highway itself so trail users don't have to cross traffic at grade. That underpass is what's driving the extended timeline. It's also why the new segment ties into the existing Greenway through the driveway and parking area directly west of the Sandstone Ranch Visitor Center, rather than through downtown Longmont.
The payoff, once it's built, is bigger than two miles of new pavement. At St. Vrain State Park, the trail is set to connect to another 2.1 miles of the Colorado Front Range Trail, which then links to the Weld Legacy Trail east of I-25 in Firestone. The Front Range Trail itself is the state's long-term plan to run a continuous trail corridor from Wyoming to New Mexico. Longmont's two miles are a small piece of that, but they're the piece that turns an in-town loop into a segment of something considerably longer.
Why the Underpass Keeps Moving the Date
The timeline hasn't slipped all at once. It's moved in the kind of small increments that add up if you've been watching the project updates rather than the planning page. A portion of the Sandstone Ranch parking area near the Visitor Center closed for the duration of the project starting in late December. By late January, the dirt trail connecting that parking area to Sandstone Drive was seeing intermittent closures for active work. Contractors hit excavating issues in late February that pushed a concrete pour back a few days and extended a trail closure that was originally supposed to end by late April.
Then came the highway work itself:
- March 23, 2026: New traffic alignment goes into effect on 119 west of the St. Vrain Creek bridge. Eastbound traffic shifts to the right shoulder, westbound moves into temporary center lanes, two lanes remain open in each direction, speed limit drops to 45 mph. This configuration was slated to run through late August.
- August 9 to 11, 2026: Overnight crews restripe the roadway and shift traffic again to move into what the city calls Phase 2 of the underpass construction, this time with concrete barriers separating opposing traffic between County Road 5 and County Road 7.
- Through winter 2026 into 2027: That Phase 2 pattern stays in place while the underpass itself gets built beneath the highway, according to the city's own project updates.
None of that is a single dramatic delay. It's a sequence of highway-adjacent construction steps that each take longer than digging trail through open ground, which is why the completion estimate has drifted a full year past what the original design timeline assumed.
What's Actually Open Right Now
If your routine already runs Golden Ponds to Sandstone Ranch and back, nothing about this project changes that route. The eight miles that make up the current Greenway aren't part of Phase 13's construction footprint. Sandstone Ranch remains your turnaround point for now, same as it's been, and the historic homestead there, built in the early 1880s by a Gold Rush-era settler, is still open to walk through.
What isn't open is the ground past it. The new mileage toward St. Vrain State Park doesn't exist as usable trail yet, and won't until the underpass and both bridges are finished. If you're parking at Sandstone Ranch specifically to access that stretch, expect to keep using the Visitor Center or Sandstone Soccer Complex lots, since the primary lot near the Visitor Center is closed for the length of construction.
One detail worth knowing if you bring a dog: the existing Greenway east of County Line Road already restricts dogs, leashed or not, for the roughly one-mile stretch into Sandstone Ranch, a rule the city adopted specifically to protect wildlife in that section. Current project materials don't specify whether that same restriction will extend onto the new mileage past Sandstone Ranch once it opens, so that's a detail to confirm once the segment is actually usable rather than something to assume either way.
If you want a different loop while this section is under construction, the Left Hand Greenway connection near Ken Pratt Boulevard gives you another direction entirely, and Golden Ponds on the west end has its own 2.6 miles of park trail that don't intersect with any of the 119 work.
The Two-Mile Gap Is a Multi-Year Piece of Something Larger
It's easy to read a highway lane shift and a fenced-off parking lot as a temporary inconvenience that'll clear up any month now. The construction updates say otherwise. The underpass beneath 119 is a piece of infrastructure with its own build sequence, and that sequence runs through this winter regardless of how the surface-level trail work is going elsewhere on site. Spring 2027 is the completion date CBS Colorado reported in March 2026, and the city's own project bulletin, which has the underpass under construction through this winter, fits that timeline. The original planning page still shows spring 2026, a full year earlier.
For anyone whose daily walk, run, or ride currently ends at Sandstone Ranch, that's the number to hold onto instead of the older estimate. The trail isn't getting longer this year. It's on track to get longer in spring 2027, once the highway underpass and the two pedestrian bridges are finished.
If you're looking at property near this stretch of the Greenway and want to talk through what a trail connection like this can mean for a home's daily use, not just its listing sheet, Jane Kraemer knows this corridor block by block and is glad to walk it with you.
