Every day, some 9,000 riders take ferries to and from Hoboken and Jersey City for the short trip to lower Manhattan. The busiest ferries in the metropolitan area, they have drawn residents and businesses to both the Financial District and New Jersey's waterfront communities.

What makes these ferries so popular? On both sides of the journey, the piers are easy to reach, close to rail and subway stations, and within short walks of office buildings. On less busy routes, the ferries launch from more remote locations, far from rail connections or areas with much foot traffic.

Mayor Bill de Blasio has announced plans for five new ferry routes in NYC. They would carry about 15,000 passengers each weekday, serving communities such as Red Hook and southeast Bronx that today have limited transit options.

Ferries will be a boon for commuters who live a long slog, or a pokey bus ride, from the subway. Whether the routes will draw as many riders as anticipated is unclear. New service along the East River has surpassed expectations, and ferries can attract residents to once out-of-the-way neighborhoods. But scores of ferry routes have been tried in New York Harbor in the past 30 years, and fewer than two dozen survived.

More important, even if all of the city's proposed routes are wildly successful, they would serve a tiny fraction of commuters. There are 30,000 trips made by ferry in our area every weekday. By comparison, 8.4 million trips are made by passengers on subways and buses.

They rely on an aging network of trains, bridges and tunnels. The system needs upkeep and modernization, along with new stations and faster buses. The MTA says it is short $15 billion for improvements.

The MTA should focus on the outer boroughs the mayor wants to help with ferries. This week, we'll release a study showing the number of commutes between outer boroughs are growing more rapidly than trips in and out of Manhattan. Many residents have to take a circuitous subway trip through Manhattan to go from, say, Brooklyn to Queens. These places need better bus, subway and commuter rail service.

Ferries can help. But in the broader picture of our transportation needs, they are a drop in the harbor.

Jeffrey Zupan and Richard Barone are transportation analysts at the Regional Plan Association, an urban policy think tank.