The acting head of the Federal Motor Carrier Safety Administration will make his first appearance before Congress on Wednesday in an important hearing for the agency. Yet Scott Darling's testimony comes as lawmakers wonder why the Dept. of Transportation can’t fill a number of key leadership positions—and as the clock is ticking on the term of his own, temporary appointment.

In a hearing Tuesday on the DOT budget, Senate Commerce Chairman John Thune admonished Transportation Sec. Anthony Foxx for the five unfilled administrator posts at DOT modal agencies. He noted specifically FMCSA, whose acting administrator “will no longer be able to serve in that capacity by the end of the month.”

And with only one nominee in the pipeline, for the Federal Transit Administration (FTA), “we can’t even begin the confirmation process for the rest of these important agencies,” Thune said. “The sooner we can get those important leadership posts filled, the better off we’re going to be.”

Sen. Steve Daines (R-MT) followed up, and questioned Foxx on the impact.

“How are we supposed to be proactive in working with these agencies when they are without appropriate leadership?” Daines asked. “What’s the barrier to filling these roles?”

Foxx defended the abilities of the acting agency heads, and added that “a lot of work is underway” on moving forward with additional nominations—but he deferred to the White House on any appointment timeline.

“We have good leaders in place, even if they’re ‘acting.’  The expectation is that there’s no drop off in our ability to focus,” Foxx said. “We do want to make sure we get the right fit for these jobs. It’s more than just trying to find somebody off the street. I do believe that, both with the folks we have in acting roles as well as any that may or not be moving through the process right now, we will keep our standards very high.”

But the Senate hasn’t been in a rush to confirm nominees, either.

Therese McMillan was re-nominated in January to lead FTA. The Senate failed to confirm her nomination during the previous Congress. McMillan, the deputy administrator, has been leading the transit agency in an acting capacity since early 2014.

The Senate did confirm Mark Rosekind as the administrator of the National Highway Traffic Safety Administration (NHTSA) late last December, amid public pressure over automobile recalls and industry oversight.

Darling was named FMCSA acting administrator in August, following the departure of Anne Ferro, who was nominated by President Obama and confirmed in November 2009. An Obama political appointee, Darling joined the agency as chief council in 2012 and has served as the agency’s top legal officer and managed FMCSA’s staff of attorneys.

He’s scheduled to testify Wednesday before the Commerce subcommittee on surface transportation in a hearing on the pending highway bill and oversight and reform of FMCSA.

Darling’s status and what might happen when his 210-day appointment expires in coming weeks remains uncertain, however. FMCSA referred questions to DOT, and a DOT spokesman told Fleet Owner that, “because these are presidential announcements,” any comment would need to come from the White House. The White House has yet to respond to a Tuesday afternoon email.

This isn’t the first time FMCSA has gone for an extended period without an administrator: Nearly 10 months elapsed between the departure of John Hill in Jan. 2009 and Ferro’s swearing in, according to an FMCSA calendar.

Otherwise in Tuesday's hearing, Foxx continued to push the Grow America Act, the administration’s half-trillion dollar, six-year transportation infrastructure funding plan. The transportation secretary emphasized that the U.S. is expected to grow by 70 million people in the next 30 years, and “for every 10 trucks now on the road, there will be 4 more by 2045.”

“A tidal wave is coming: It’s a tidal wave of people, of passengers and of goods,” Foxx said. “Are we going to choke on own growth or are we going to build for it?”

Foxx also noted that the problem is a safety issue, as well as one of capacity.

“The roads won’t expand by themselves to relieve congestion,” he said, and called on Congress to show leadership and work with the administration. “The need for a long-term bill cannot be overstated: It will help get us back into action mode again. I’m a secretary, not a magician. I can’t make stuff happen without the resources to do it.”