A plan to complete a network of nearly 900 miles of trails connecting the Washington region could be accomplished with an investment of just over $1 billion, according to a coalition of nonprofits, business and government groups.

The estimate, from the Capital Trails Coalition, would build 408 miles of trails that are planned across the region, nearly doubling the size of the existing network. The proposed trails would fill gaps in a system of nearly 470 miles, improving connectivity between neighborhoods and public transit, commercial districts and jobs, the group said.

The coalition of more than 60 organizations, including government agencies, environmental groups and business improvement districts, is pushing for most of the projects to be completed over the next decade, though funding has only been secured or identified for about 10 percent of them.

The proposed bike and pedestrian paths are already in the plans of transportation, parks and planning departments in the District and the city of Alexandria as well as Arlington, Fairfax, Montgomery and Prince George’s counties. Over half of the new trails, 236 miles, would be in Prince George’s.

“Building this whole network of trails will cost less than it takes to pay for one highway,” said Leah Gerber, trail planner coordinator with Rails-to-Trails Conservancy, a nonprofit member of the coalition. “And the benefits will be felt across the entire region.”

The idea appears to have gained political support across the region as jurisdictions seek ways to reduce congestion and their carbon footprint, and as more people turn to biking and walking for commuting and recreation. The region’s Transportation Planning Board recently adopted the plan.

The challenge is bringing the more than 150 projects in planning to construction. Funding for such projects is limited, and they have to compete with transportation projects that historically have taken priority, such as fixing roads. In some jurisdictions, projects have stalled over decisions about which agency would maintain the facilities once they are built.

D.C. Council member Charles Allen (D-Ward 6), vice chairman of the Transporation Planning Board, said that while $1 billion may seem like a lot of money, it’s a necessary investment.

“We invest in mobility. We invest in the ability to make sure that people get around in our region,” Allen said. “We certainly spend a lot of money on our streets, on our sidewalks, on our alleys. We spend a lot of money on Metro. So I am not going to have sticker shock for the fact that it will cost money to invest in this entire network of trails.”

When considering trails and cycling as part of the commuter network, it puts that price tag in context, said Katie Cristol, a member of the Arlington County Board.

“As we all embrace the need to move more people around the region without single-occupancy cars, the opportunity to invest in another form of infrastructure that enables that, like trails, is going to become more and more attractive,” said Cristol, who is vice chair of the Northern Virginia Transportation Commission.

It only makes sense, she said, to tackle the planning and funding as a region, just as the District, Maryland and Virginia have come together for other regional transportation priorities such as Metro.

Katie Harris, trails coalition manager with the Washington Area Bicyclist Association, said the group is working to improve regional coordination to complete the Capital Trail Network and ensure that when local governments discuss transportation funding, the conversation isn't just about road infrastructure and transit.

“It is important to elevate these trail projects as the true transportation assets that they are and have jurisdictions acknowledge and fund them in ways that are competitive with other improvements to our roadway system,” Harris said.

Trails, Harris said, could have as much or greater impact on the region’s mobility as other more costly projects. For comparison, the coalition cites the widening of 22.5 miles of Interstate 66 in Northern Virginia to add toll lanes, at a cost of nearly $4 billion. Maryland’s proposed widening of the Capital Beltway and portions of Interstate 270 has a price tag above $9 billion. And, at a recent trails summit, participants said the region could pay for the trail network for almost the same amount it will cost to rebuild and widen the American Legion Bridge.

Expanding the trail system offers a solution to the region’s traffic congestion while also promoting health, a cleaner environment and economic savings, proponents said.

A recent report by the Rails-to-Trails Conservancy estimates that the region would record $500 million a year in health-care cost savings if it completed the network.

With those annual benefits, the network “pays for itself many times,” Torsha Bhattacharya, director of research at the conservancy, told advocates, business leaders and government officials at the recent summit. The conservancy led the coalition’s research on cost and has done extensive work on trail projects across the country.

Research also shows that a more robust trail network leads to greater use. A 2015 report by transportation researchers at North Carolina State University concluded that completing a critical link of a trail in Durham led to an additional 300,000 trips, or a 133 percent increase in use. In Ohio, the Central Ohio Greenways network concluded that connecting the Alum Creek Trail led to a 40 percent increase in activity.

Trail use in the Washington region is already strong. The share of bike commuting has increased in recent years and is expected to keep growing, particularly as more jurisdictions open their trails to e-bikes, as some have recently done. Some of the most popular trails connect neighborhoods in suburban Washington to the city and offer access to transit stations, businesses and recreational facilities.

The Capital Crescent Trail, the busiest in the region, links downtown Bethesda to Georgetown and has more than a million users a year, according to counts by the Capital Crescent Trail Coalition. In Northern Virginia, the Four Mile Run and Custis trails each have about 500,000 users annually, touch on major commercial districts, and link to other trails leading to downtown Washington and Reagan National Airport.

The Capital Trails Coalition, founded in 2015, has prioritized 11 trails it says are the most critical to improving connection and access to underserved neighborhoods. Some of the projects would eliminate gaps that prevent users from continuing trips across jurisdictional lines.

Among those projects is an extension of the Capital Crescent Trail with a 4.3-mile section in Montgomery that will connect downtown Bethesda to downtown Silver Spring. That project is funded as part of the light-rail Purple Line project and is under construction.

Other projects include: incomplete segments of the Metropolitan Branch Trail, which connects downtown Silver Spring to Union Station; a 6.4-mile extension of the WB&A Trail in Prince George’s that would bring the trail from Annapolis Road to the county’s border with the District in an area underserved by trails; and about five incomplete miles of the Arlington Boulevard Trail, a project that is partially funded and needed to improve pedestrian safety in the Route 50 corridor.

Less further along is a plan to build a trail as part of the reconstruction of Long Bridge. The reconstruction project, which would expand rail capacity over the Potomac River, is in the planning phase, with funding only to cover some design and engineering. It includes a stand-alone bike and pedestrian bridge to be built upstream from the new bridge. The trail would extend from Crystal City to L’Enfant Plaza, allowing people to walk or bike across the Potomac between the D.C. waterfront and Crystal City, home to the new Amazon headquarters and an anticipated 25,000 jobs.

The trail “would unlock the ability for people to get to jobs without a vehicle,” said Joe McAndrew, director of transportation policy at the Greater Washington Partnership, a group of corporate leaders that has been lobbying for more transportation alternatives in the region. “This is a win-win for our bicycling and walking and train connectivity.”

As visionary as it is, the regional trails plan also is an important road map in prioritizing trail projects and identify funding sources, said Bob Patten, a planner with the Prince George’s Department of Parks and Recreation, which has mapped out more than 200 miles of new trails.

Among the options that the coalition is studying to jump-start projects is getting the public sector more directly involved in trail building and turning to referendums to give voters a say in funding the projects.

“Many communities that have put popular public benefits into revenue referendums have passed them, and then you have clear voter support,” Patten said.

Moving forward, the Capital Trails Coalition says it plans to build on the support by further studying the health, economic and environmental benefits of the trail network to make its case for the investment.

“Biking and walking is fun and people like to do it,” Greg Billing, executive director of the Washington Area Bicyclist Association said at the trail summit last month. The main reason people don’t walk or bike, he said, is because they don’t have access to a system.

“We are not trying to convince people. We are trying to make it accessible,” he said.