Pull up the trail conditions for Four Mile Run Trail on AllTrails and you'll see a note dated February 2026: a section between Mount Vernon Avenue and South Lang Street is closed for construction, detour in place. If you've actually run, biked, or walked a dog through that stretch this summer, the note is technically still true and also several months behind what's happened on the ground. The detour is real. The story behind it has moved faster than the app has bothered to update.
Here's what's actually going on, and why the timeline matters if you use this trail the way most of South Arlington does: as the connective tissue between Shirlington, Barcroft Park, Glencarlyn Dog Park, and the Mount Vernon Trail that carries you into Old Town or across the river into DC.
What the Detour Signs Are Actually For
The closure sits at one specific pinch point: the trail underpass beneath the Arlington Ridge Road and Mount Vernon Avenue bridge, on the Arlington County side, just north of the span itself. That underpass has been closed since construction started, and trail traffic gets rerouted onto South Lang Street and South Glebe Road to reconnect on the other side. Signs mark the route. It's not elegant, but it's not a mystery either once you know what's causing it.
The bridge itself is the actual project. Arlington County and the City of Alexandria are jointly reconstructing the Arlington Ridge Road/Mount Vernon Avenue bridge, replacing a deteriorated roadway and the bridge deck underneath it. Construction started in summer 2025 and the full project isn't expected to wrap until summer 2027. Crews have spent this year on Phase 1, which means demolishing and rebuilding the west side of the bridge over Four Mile Run. That's the phase that took the trail underpass down with it.
The bridge itself has kept operating the whole time, just with reduced capacity. One lane in each direction stays open on the east side, and the east sidewalk has remained open to pedestrians throughout. It's only the trail underpass, the piece that lets you stay on Four Mile Run Trail without popping onto surface streets, that's been out since the project began.
The Girders Beat the Schedule
If you want a read on how this project has actually been going, look at what happened in May. The county scheduled off-peak lane closures from May 16 through May 23 so crews could crane five steel girders into place over the stream, with work windows running 9 a.m. to 3 p.m. on weekdays and 9 a.m. to 5 p.m. on Saturdays, no work on Sunday the 17th.
Crews finished early. All 15 girders for the span went in ahead of schedule, and the county canceled the remaining closure days, restoring normal bridge traffic before Memorial Day weekend. That's the kind of detail trail apps don't track but neighbors who drive or walk past the site every day noticed immediately.
None of that changed the trail underpass status. Girder installation was bridge work, not trail work, and the underpass stayed closed through the summer regardless. But it does tell you something about how this particular contractor crew has been running: not behind, and in at least one case, notably ahead.
What's Actually Still Open
It's worth being precise here, because the AllTrails note reads like the whole trail took a hit and that's not what happened. Four Mile Run Trail runs seven miles from Benjamin Banneker Park in Falls Church down to where it meets the Mount Vernon Trail near Reagan National Airport. The closure affects one underpass near the Arlington Ridge Road bridge, not the trail's connection points further north.
Bluemont Junction Park, where the Bluemont Junction Trail branches off toward Ballston, is untouched. So is the stretch through Barcroft Park and the access point to Glencarlyn Dog Park, both well upstream of the construction zone. If your regular loop uses Four Mile Run Trail to link into the W&OD Trail or the Custis Trail, the combination locals call the Arlington Loop, you're not rerouting your whole run. You're rerouting one bridge crossing near Shirlington, using South Lang Street and South Glebe Road, and picking the trail back up on the other side.
For dog walkers headed to the Shirlington dog park off the trail, or anyone using the Jennie Dean Park area nearby, the same logic applies. The detour is a few blocks of street walking around one construction zone, not a closure of the corridor.
What October Actually Buys You
Here's the part that should change how you think about the rest of this year. Arlington County's own project page has put Phase 1, the phase responsible for the trail underpass closure, on track to finish in October 2026. That's not a distant milestone. That's this month or next.
Given that the same project already beat its girder-installation deadline by roughly a week, treating October as a soft ceiling rather than a hard one seems reasonable. Whether the trail underpass reopens the day Phase 1 formally closes out or trails the paperwork by a few weeks, the detour that's been sending runners onto South Lang Street since AllTrails last updated its notes is closer to its end than its beginning.
The full bridge project runs through summer 2027, so don't expect the reconstruction to disappear entirely. But the piece that affects your daily use of the trail, the underpass and its detour, is tied specifically to Phase 1, and Phase 1 is the part with an October date attached to it.
Why the County Bothered
The Arlington Ridge Road bridge sits inside the broader Four Mile Run Valley planning area, which stretches from Barcroft Park down toward I-395 and covers everything from trail realignments to how Jennie Dean Park might eventually expand. The bridge project's stated goals include returning the span to safe operating condition and improving access for people walking, biking, and driving, which is county language for making sure this exact kind of trail connection doesn't stay broken indefinitely once the construction wraps.
It's a reasonable trade for anyone who uses this stretch regularly: a few more weeks of a street detour in exchange for a rebuilt crossing that's supposed to serve trail users better once it's done. If you want the specifics straight from the source, Arlington County's Four Mile Run Valley project page lays out the planning documents, and Alexandria Brief's coverage of the early girder finish has the most recent on-the-ground update.
For now, if you're heading out for a run along Four Mile Run this fall, the detour signs near the Arlington Ridge Road bridge are still worth expecting. Just don't assume the trail app's February timestamp is the last word on where things stand.
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