"road metal" meaning in English

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "road metal (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
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          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Roads",
          "orig": "en:Roads",
          "parents": [
            "Road transport",
            "Transport",
            "All topics",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc.; gravel; blue metal."
      ],
      "id": "en-road_metal-en-noun-LFyKb0YJ",
      "links": [
        [
          "gravel",
          "gravel"
        ],
        [
          "blue metal",
          "blue metal"
        ]
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "road metal"
}

{
  "head_templates": [
    {
      "args": {
        "1": "-"
      },
      "expansion": "road metal (uncountable)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
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        "English lemmas",
        "English multiword terms",
        "English nouns",
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        "en:Roads"
      ],
      "glosses": [
        "Hard dark stone crushed into small pieces used in surfacing roads, for ballast in railway tracks, etc.; gravel; blue metal."
      ],
      "links": [
        [
          "gravel",
          "gravel"
        ],
        [
          "blue metal",
          "blue metal"
        ]
      ],
      "tags": [
        "uncountable"
      ]
    }
  ],
  "word": "road metal"
}