In a move that could shift the dynamics of carpools across the Denver metro area, the popular High Occupancy Vehicle rules that let two occupants ride free on Express Lanes through often-congested segments of U.S. 36 and Interstate 25 will require at least one more rider to claim the privilege.

The change has a name: HOV 3. And it’s been four years coming, even though many commuters may be surprised to learn that it takes effect Jan. 1.

The move, designed to ease congestion in high-traffic times and also chip away at a transportation revenue shortfall, affects the Express Lanes along Interstate 25 in Denver and along U.S. 36 between Boulder and Denver.

Vehicles that try to exercise HOV privileges with only two occupants after Jan. 1 will be subject to enforcement that can include a $250 fine. The state hopes the new rule will encourage commuters to take steps to ensure the quickest and least expensive travel, which could mean taking a bus, such as the Flatiron Flyer on U.S. 36, that has access to the express lanes; paying the premium to use the Express Lanes; or finding another rider or two to comply with the free HOV conditions.

“With our growing population, we’re going to need to make sure HOV lanes on the express lanes are able to provide reliable travel time, especially when you look at the projected growth for this state,” said Megan Castle, spokeswoman for the Colorado Department of Transportation. “The other component is that the financial aspect can’t be ignored. We need funds that come from additional toll revenue to offset the cost of infrastructure, adding that new express lane and also for ongoing operations and maintenance.”

For those whom Castle calls HOV “purists,” there are a couple of options that could make commuting easier. CDOT is offering “switchable” transponders that can be used on their regular setting to pay Express Lane tolls, but then switched to HOV mode when there are at least three occupants and ride the Express Lanes for free.

Motorcyclists have always been able to use Express Lanes for free with a sticker indicating they have an account, but starting in mid-October the rules changed to allow them to ride for free without the sticker.

Among 10 states that use HOV lanes, Colorado is the seventh to adopt the three-person rule, Castle said.

Regional Transportation District spokesman Scott Reed noted that already there has been an uptick in ridership on the Flatiron Flyer from Boulder to Denver and anticipates that I-25 bus routes that use the Express Lanes could see a similar rise with the change in the HOV rule.

“We think this will be additional incentive for people to explore transit alternatives,” Reed said. “I think CDOT has done a good job advertising the fact that this is taking effect at the first of the year. But there are always some people surprised regardless of what the change might be.”

The original agreement came together in 2013, with the Jan. 1, 2017, date set for the conversion to HOV 3. But neither CDOT nor other transportation advocacy groups, such as the nonprofit 36 Commuting Solutions, figured that people would remember without an extended education campaign.

“Even though the financial agreement was four years ago, I don’t know that the public pays attention to something that happens four years later,” said Audrey DeBarros, executive director of 36 Commuting Solutions. “That’s why we’re teaming up with transportation partners to get out the word, so people aren’t surprised and can see resources to find a third carpool partner or other transit options.”

She said her group encourages commuters to obtain the switchable transponder, “by far the most cost-effective way to utilize HOV Express, whether solo or as a carpooler, and also to find that third person so they can cruise free under HOV rules.”

DeBarros said her organization also will be offering incentives for people currently driving solo on U.S. 36 such as free RTD ticket books good for 10 rides, or a $75 subsidy for new car or van pools.

“So it’s meant to be a carrot,” she said, “another way we’re trying to ease the anticipated change.”

Smart Commute Metro North also has similar incentives available for those commuting on I-25. Resources such as Mywaytogo.org, an online ride-sharing service of the Denver Regional Council of Governments, also can help commuters find partners for the ride.

“We’re making some changes to our trip-planning platform to facilitate easier formation of three-person carpools,” DRCOG spokesman Steve Erickson said. “For a lot of people, it’s going to be a bit painful. It’s one thing to find one person, but it can really be a challenge to find another. The website is making that easier to do.”

The platform is expected to be up and running within the next few weeks.

In toll lanes, the prices will remain variable depending on time of day as a means of regulating the traffic flow and ensuring optimal travel times.

Boulder County commissioner Elise Jones describes the public education effort as “a work in progress,” and that ongoing measures to get the word out loom critical so people aren’t surprised in January.

“If we step out of our individual daily commute and look at what’s good for the region, maximizing mobility is the name of the game,” Jones said. “The more people who can travel along that corridor with minimal congestion, the better. While I think the switch to HOV 3 is a challenge, it’s also an opportunity.”