Calculate duration for route through ferry

curl -gX GET 'https://router.hereapi.com/v8/routes?'\
'origin=51.13747,1.33148&'\
'destination=50.96035,1.905025&'\
'return=summary&'\
'transportMode=car&'\
'apiKey=YOUR_API_KEY'

{
  "routes": [
    {
      "id": "fe1d3ac8-411e-4b47-bd2a-8056484865ce",
      "sections": [
        {
          "id": "3f104c2d-7ec2-4a63-9e3b-dcf6b9acd251",
          "type": "vehicle",
          ...,
          "summary": {
            "duration": 541,
            "length": 3898,
            "baseDuration": 501
          },
          "transport": {
            "mode": "car"
          }
        },
        {
          "id": "2239e39b-4eef-4ce0-9988-1c434a1e13a9",
          "type": "transit",
          "preActions": [
            {
              "action": "board",
              "duration": 1200
            }
          ],
          "postActions": [
            {
              "action": "deboard",
              "duration": 600
            }
          ],
          ...,
          "summary": {
            "duration": 5449,
            "length": 40565
          },
          "notices": [
            {
              "title": "No schedule information available.",
              "code": "noSchedule",
              "severity": "info"
            }
          ],
          "transport": {
            "mode": "ferry",
            "name": "Dover-Calais"
          }
        },
        {
          "id": "554714bc-a8c1-406f-8895-d2f81f88722e",
          "type": "vehicle",
          ...,
          "summary": {
            "duration": 440,
            "length": 5273,
            "baseDuration": 396
          },
          "transport": {
            "mode": "car"
          }
        }
      ]
    }
  ]
}