There was a time when hitting the road at 4 a.m. meant a quiet commute, free of traffic and brake lights. On the Bay Area's busiest bridges, not anymore.

Not even close.

Data from the Metropolitan Transportation Commission shows early morning traffic on Bay Area bridges has spiked dramatically over the past two years. The number of drivers who cross the Bay Bridge at 4 a.m. has gone up a whopping 36 percent since the 2014-2015 fiscal year.

On the San Mateo Bridge, the Bay Area's second most-popular state-run bridge, that number for the same early hour has gone up 31 percent. Not to be outdone, the Dumbarton Bridge has seen a 39 percent jump in ridership in the last two years.

"It's no surprise as the Bay Area economy has been growing for quite a few years now, the commute has got longer, both in the morning and the afternoon," said MTC and Bay Area Toll Authority senior public information officer John Goodwin. "We see this real clearly in the wee small hours of the morning."

VIDEO: Could higher tolls mean less traffic? (Story continues below)

Studying the month-by-month numbers shows it's not just that numbers across the board are rising — though, they are — but the number of people on Bay Area roads and bridges at 4 a.m. is rising much faster than any other time period.

For example, in the same two-year time frame on the Bay Bridge that there was a 35 percent increase in drivers at 4 a.m., there was a marginal decrease in drivers who crossed at 6 a.m. (1.5 percent fewer last year than two years prior). The number of drivers on the San Mateo Bridge at 6 a.m. similarly held steady (just a 0.1 percent decrease), while the 4 a.m. numbers spiked.

"We, as a region, are clearly getting on the move earlier than we used to," Goodwin said. "This is not just a Bay Bridge phenomenon. It's also happening with the Carquinez and the Benicia bridges."

MTC's inspiration for tracking the early morning numbers came from a KCBS reporter who noticed his drive to work between 4 a.m. and 5 a.m. was starting to look increasingly crowded. The data collected between July 2014 and June 2017 backed up that reporter's hunch.

But tracking down the reason behind the trend isn't as easy. Goodwin doesn't think there is a single answer.

"One could point to the $6 toll that begins at the Bay Bridge at 5 a.m. That certainly is an incentive for some number of people who are driving across that bridge to do so before 5 o'clock," when the toll is only $4.

However, because there has been an increased in ridership across other area bridges that don't use congestion pricing, that can't be the only factor at play.

"Mostly I think it comes down to the dynamic state of the economy," said Goodwin. He believes growth in Bay Area tech has also created the need for those in the construction, janitorial and food service industries to start earlier to support tech workers and tech companies.

As for possible solutions, MTC and the toll authority are considering several options. Increasing the number of drivers using Fastrak would reduce backup at the toll plazas (right now only 65 percent of drivers crossing the Bay Bridge during commute hours use Fastrak). The agency could also expand the congestion pricing structure currently in place on the Bay Bridge to other heavily trafficked bridges.

MTC is also proposing a toll increase of $2 to $3 on all seven Bay Area state-run bridges, but Goodwin said that plan is still in the early stages of development and it's too soon to say what impacts it would have on traffic.

"It's a difficult puzzle in a region as large as ours and one that depends on our toll bridges as much as we do," Goodwin said. "It's a tough nut to crack."

Read Alix Martichoux's latest stories and send her news tips at amartichoux@sfchronicle.com.