Run the Dissolve tool, where your Dissolve field is road type (i.e. dirt, asphalt, etc.). If you allow multipart features you will get a single feature for all the asphalt, another feature for all the dirt, etc. This means you want to make sure to allow for singlepart features.
You may want to delete the dirt features or other non bike-rideable roads. If you ...
When one creates a join and links an existing shapefile attribute table with another table, do export the two joined table as a new shapefile and load it again into QGIS. The joined table's fields will then be accessible to various QGIS tools, e.g., dissolve.
I think I found a solution based on this past post. Full code:
//Create demo data//////////////////////////////////////
var region = ee.Geometry.Rectangle(-120, 34.669, -99.536, 50.064);
// Create 10 random points in the region.
var randomPoints = ee.FeatureCollection.randomPoints(region, 1000);
//Produce 10 km buffers, these are the polygons we want to ...