Financed by JICA it is a project of Sri Lanka's Road Development Authority.
It uses cable stays connected to two towers at the ends to hold up the bridge with no piers in the middle of river. But unlike a conventional cable-stayed bridge, the design, known as an extradosed bridge uses shorter towers and a thicker deck.
"Using this design we can avoid putting any piers on the Kelani River," Namal Ralapanawa, senior project specialist at JICA said.
"That was one of the main concerns environmentally. The river is quite slow at that point and quite prone to floods. So we wanted to make sure that the bridge will not hinder the water flow."
The 6-lane bridge is 380 meters long over the river and with the elevated approach roads it will be over one kilometre long.
JICA says it now takes eight minutes to travel two kilometres over existing bridges to Orugodawatte during the morning and evening rush hours. It will increase to 20 minutes by 2020 if there is no solution.
Speeds will fall to between 5 to 10 kilometres an hour or 'jogging speed'. But the bridge is expected to reduce the time to traverse the two kilometer stretch to 4 minutes and at speeds of around 20 to 45 kilometres an hour.
JICA chief representative Kiyoshi Amada said Japan was continuing to finance Sri Lanka despite cuts in the budget for overseas development assistance over the years.
"Sri Lanka is one of our important partners," he said. "So we have not decreased assistance to Sri Lanka.
He said there may have been a decrease in grant funding. But even if budgetary resources were tighter, JICA was itself financing some of the lending through borrowings.
The bridge loan was also a 'very soft loan' terms of 0.1 percent interest and 40-year repayment, Amada said.
JICA officials said about 440 households displaced from the project will be given alternative housing. Some of them were living in areas prone to frequent floods.
In Sri Lanka there have been protests over evictions of residents for urban development purposes.
Under JICA procedures and Sri Lanka's policy on involuntary resettlement policy they will be given cash compensation or alternative housing.
About 90 percent of the householders were 'non-title holders' they would be given facilities to lead 'equivalent or better lives', Ralapanawa said.
Sri Lanka's Road Development Authority will purchase a housing project from the urban development authority to be distributed to the displaced families following consultations which have already begun, she said.
Title holders will get the option of the same package plus additional compensation, or cash compensation she said.
Update II/Corrected JICA chief representative Kiyoshi Amada