More than 150 years ago, the battles of Bull Run were fought in the fields and hollows along the Warrenton Turnpike. Twice in 13 months, Union and Confederate soldiers spread out in battle lines on the sides of this historic roadway and engaged in Civil War clashes that helped shape the course of history.

Brave Americans consecrated this land with their blood, and subsequent generations have honored that memory. For 60 years, activists have fought off numerous attempts to mar the landscape with inappropriate, encroaching development, including a freeway, a giant cemetery and a 1.5 million-square-foot mall. Even Disney was rebuffed in its bid to build a heritage-themed park nearby.

But just as the battlefields’ ease of access to Washington via major 19th-century roads allowed curious picnickers to watch the smoke rise from the first Battle of Bull Run, so too has this proximity doomed Manassas National Battlefield Park to be one of the country’s most endangered battlefields.

Today, and for many years, the chief threat has been choking commuter and industrial traffic through the heart of the park. The old Warrenton Turnpike, now Lee Highway (Route 29), features bumper-to-bumper congestion as it runs through both battlefields. Park officials and supporters have thus far resisted widening the road beyond two lanes through the battlefield and losing historic land in the process. But this causes a funnel of congestion within the park, particularly at the intersection with Sudley Road (Route 234).

I am not a municipal planner; my expertise lies in history and the park’s visitor experience. But as someone who has studied and explored every inch of these battlefields, I have watched the debate over the proposed Bi-County Parkway with intense interest. Good points have been made by both sides about the project’s potential benefits and side effects. But the debate has lost sight of a very real and ever-worsening problem: Commuter and industrial traffic threatens to eclipse this national park, one of America’s crown jewels.

Suburban growth in the region has accelerated to the point that an estimated 26,000 vehicles squeeze through the park each day. The crisis is most visible at the intersection of routes 29 and 234, in front of the Stone House, a key landmark of both battles. Here, commuters and battlefield visitors alike must sit through multiple cycles of the stoplight, burning gas and patience. The incessant, deep-throated rumble of dump trucks from two nearby quarries, plus other industrial vehicles, obliterates any chance of reverie. A 2005 Environmental Impact Statement by the National Park Service noted that the roughly 10 percent density of construction traffic on these roads is up to four times heavier than what similar thoroughfares typically experience. The historic buildings’ floors vibrate noticeably underfoot, and cracks have appeared in the structures.

Manassas arguably experiences worse traffic through its heart than any other national park. Visitors traveling the designated tour route must fight their way through the Stone House intersection eight times — an excruciating experience for even the most patient traveler. I have led many tours where traffic noise made it impossible to effectively communicate at key spots.

For decades, government officials have reached for solutions. The 1988 congressional legislation that protected Stuart’s Hill already called for the same sections of routes 29 and 234 discussed in the Bi-County Parkway proposal to be turned over to the National Park Service. Clearly, a permanent answer is long overdue, yet practical ideas have rarely gotten beyond the drawing board. And recent “improvements” — such as the addition of traffic lights and lengthy turn lanes at the Stone House intersection — have only made traffic worse.

This is why I believe the Bi-County Parkway proposal deserves a second look. The plan calls for the transfer of Route 234 to the National Park Service to become a park road, but only after construction of the Bi-County Parkway is complete. It partially addresses congestion on Route 29 with traffic-calming measures that would bring traffic speeds into line with the surrounding national park setting.

As a longtime advocate of Manassas National Battlefield Park and a participant in its many preservation battles, I have stood shoulder-to-shoulder with some of the proposal’s opponents. However, in this instance, I believe we cannot ignore the very real threat to this national shrine posed by continued inaction.

The truth is, as the commuter traffic in the park escalates, the clamor to widen the roads through the park will inevitably increase. Opinions differ on the proposed Bi-County Parkway, but one thing is certain: It is the only viable plan on the table that addresses the park’s traffic burden, provides a framework for constructive dialogue on park traffic and acknowledges that something must be done. The Bi-County Parkway may not be the ideal solution, but it is a start. If action is not taken now, we may miss our last chance to prevent irreparable harm to a national treasure.

The writer is chief historian emeritus of the National Park Service and a member of the Civil War Trust board of trustees.