text: var turf = require('turf')
  
  module.exports = function (sources, tile, write, done) {
    var layer = sources.osmdata.osm
    var lengthOfRoads = 0
  
    for (var i = 0, j = layer.features.length; i < j; i++) {
      var feature = layer.features[i];
      if (feature.properties.highway && feature.geometry.type === 'LineString') {
        lengthOfRoads += turf.lineDistance(feature)
      }
    }
  
    done(null, lengthOfRoads)
  }
  