In a bid to spur a legal reckoning with its critics, the Northern Virginia Transportation Authority essentially sued itself last month and a court set Aug. 16 as the deadline for potential opponents to come forward and file motions in the case.

The idea was to force them to take their best legal shot and thereby have the work of the newly revived regional transportation body stand or fall — quickly — so the vast task of easing congestion could proceed without a cloud of uncertainty.

A longtime critic of the authority, Del. Robert G. Marshall (R-Prince William), obliged Thursday, filing a legal challenge in Fairfax County Circuit Court ahead of Friday’s court-ordered deadline.

“They weren’t following the law,” Marshall said of authority board members, who he accused of “thumbing their nose at the legislature.”

That contention will be tested next month. Marshall was behind an earlier, successful legal challenge to the authority. In 2008, Virginia’s highest court stopped the authority from raising taxes to fund projects, saying it had no constitutional right to do so. This time, it’s the legislature that raised the taxes — which is one of its clearest powers — so Marshall is raising a narrower question.

Del. Bob Marshall (R-Prince William)

The legislation approved in Richmond this year requires that the authority “shall give priority to selecting projects that are expected to provide the greatest congestion reduction relative to the cost of the project and shall document this information” in every case.

Marshall says the authority didn’t do that in choosing its first batch of projects. The authority says it did. And now they’re headed to court. Fairfax County jumped into the case as a plaintiff as well, siding with the authority.

Del. Joe T. May, a Loudoun Republican and member of the transportation authority, defended the way the authority chose the projects, which totaled more than $209 million.

May, an engineer and inventor, often wrestles with complex technical questions. He said the authority was certainly not “thumbing our nose” at the General Assembly. Marshall, May said, is in essence arguing that there was a mathematical equation available to authority members to determine which projects would cut congestion the most.

“There isn’t an equation available,” May said. “Bob’s comment is a little bit like, ‘Listen, we ought to spend our money more wisely.’ Okay, great idea. Why don’t you tell me exactly how to measure that?”

Marshall said the authority should have waited to start spending its money until such a measurement tool was available. Already, the legislature has instructed the Virginia Department of Transportation to come up with an “objective, quantitative rating for each project according to the degree to which the project is expected to reduce congestion.” But the first ratings won’t be out until December 2014, according to VDOT.

Other authority officials said their project list, years in the making, already relies heavily on qualitative analysis and also must take into account factors including geographic diversity and construction speed.

Marshall cited the example of a rural stretch of Route 28 in Prince William County near Nokesville, which the authority is planning to expand, as proof of misplaced priorities.

“Fixing a section of Route 28 where you have cows and alpacas viewing the cars that go by is not congestion mitigation. It’s a waste of money,” Marshall said.

May said with that kind of thinking, many roads, including the Pennsylvania Turnpike, which traverses areas with few residents, would never have been built. It’s “easy to find apparent paradoxes,” May said, but you have to look at the big picture. “The traffic may be originating somewhere else, and the solution lies in widening an area that’s out in the country.”