Hillsborough Tartan Trail Road Closure and Detour Update

Anyone who drives Tartan Trail Road toward Crystal Springs Road this year has already met the barricades. Orange signage, a detour arrow pointing toward Black Mountain Road, and a bridge over San Mateo Creek that has been closed to through traffic for months. If you live nearby, you have adjusted. You leave five minutes earlier, or you have learned the back way through Skyline Boulevard without thinking about it anymore.

Here is the part most residents do not know, and it explains why this repair has taken so long: the Town of Hillsborough does not actually own that bridge.

A Bridge On Someone Else's Land

The Tartan Trail Road Bridge sits on land owned by the San Francisco Public Utilities Commission, inside unincorporated San Mateo County, technically outside Hillsborough's own town limits. It functions as a critical crossing for people who live here, but on paper it belonged to nobody in particular. When erosion, pavement failure, and subsurface damage linked to a failed PG&E conduit trench started compromising the roadway, there was no obvious agency positioned to step in and fix it.

Hillsborough did anyway. After confirming it had the legal standing to act despite the bridge sitting outside its own boundaries, the Town launched an expedited procurement process in the fall of 2025 rather than waiting through the years-long design and permitting cycle that normally governs a bridge project. The Town's own project page describes it as an emergency design-build approach built specifically to compress that timeline. On November 10, 2025, the Town Council awarded the repair contract to Power Engineering Construction Company, and construction was slated to begin as early as June 3, 2026.

That jurisdictional gap is worth sitting with for a second. A road that hundreds of households rely on to get in and out of town sat in a kind of administrative no man's land until Hillsborough decided the fix mattered more than the fine print of who technically owed it.

What The Detour Actually Looks Like

While construction continues, there is no through access between Tartan Trail Road and Crystal Springs Road. The closure runs 24 hours a day, and the Town has posted a specific detour route for the duration:

• Crystal Springs Road
• Skyline Boulevard
• Golf Course Drive
• Hayne Road
• Black Mountain Road

The project's timeline, as posted by the Town, anticipates the closure lasting through Winter 2026. Given that construction was set to start as early as June of this year, that means if you are reading this in August, the detour is almost certainly still your daily reality, not a temporary inconvenience that has already resolved itself.

The Town's own language calls the crossing "a critical access point into Town for many residents," which is a fairly clinical way of describing what is, in practice, someone's commute, a school pickup route, or the way a household gets groceries home.

The Trail You've Been Driving Past

Here is where the inconvenience turns into something useful. Part of the detour runs along Crystal Springs Road toward its junction with Skyline Boulevard, which happens to be one of the main access points for the Sawyer Camp Trail, the most heavily used segment of the Crystal Springs Regional Trail system. If your usual walking loop near Tartan Trail Road has been disrupted, you are now driving right past a better one.

Sawyer Camp Trail is fully paved and managed by San Mateo County, and it draws roughly 300,000 visits a year, which makes it one of the more heavily trafficked recreational corridors on the Peninsula, not a quiet backup option. It runs along the Crystal Springs Reservoir for most of its six miles, and about halfway along you reach the Jepson Laurel, the largest and oldest California laurel in the state. At its northern end, the trail connects across the Crystal Springs Dam Bridge, which opened in January 2019 and links into the rest of a regional trail network that eventually reaches 17.5 miles from San Bruno down to Huddart Park in Woodside.

South of Highway 92, the character changes. That stretch, the Crystal Springs segment proper, is unpaved dirt, open only to walkers and horses, and closed to pets under county rules. It passes the Pulgas Water Temple and runs near Filoli and Edgewood Park, which makes it a different kind of walk entirely, quieter and slower, better suited to an unhurried Saturday than a quick loop before work.

Neither of these was created because of the bridge closure. They have been there for decades. What changed is that the detour now puts them directly in your path instead of somewhere you have to plan a special trip to reach.

A Town That Runs On Both Kinds Of Infrastructure

Hillsborough is a place that can host the 70th running of the Hillsborough Concours d'Elegance on the fairways of Crystal Springs Golf Course one June and, the same year, be knee-deep in an emergency repair on a bridge that technically belongs to San Francisco's water utility. Both things are true of the same small town, and neither cancels the other out. The car show gets the attention. The bridge gets fixed anyway, jurisdiction or not, because residents need the road.

If you are one of the households navigating the Black Mountain Road detour this fall, that context will not shorten your drive. But it might explain why a repair that sounds simple from the outside has taken the better part of a year to resolve, and it might send you toward a trailhead you have been driving past without stopping.

For more on what makes this town run, and what it is like to actually live here beyond the headlines, the team at LeDoux Group has spent years paying attention to the details that matter to Hillsborough residents, on the road and off it.