Speed values in Mapzen Mobility APIs

To calculate speed and related factors when routing, such as travel times, Mapzen Mobility APIs consider speed limits in the OpenStreetMap source data, defaults for a particular category of road, or a measure of whether the road is in an urban or rural environment.

Real-time or historical traffic information is not currently included in speed calculations. Mapzen is working towards these capabilities, and the tiled data structures of Valhalla (the open-source project that powers the Mobility APIs) and dynamic costing approach can readily support traffic information when available.

Assignment of speeds to roadways

Routing data contains two attributes to denote speed: speed and speed_limit.

The most important for routing determination is speed, given in units of kilometers per hour. The speed value, along with the length of the roadway edge, determine the travel time along a road section.

The speed_limit contains the posted speed limit, if available, and can be used by mobile navigation applications to display the speed limit and possibly alert the driver when it is exceeded.

The speed is assigned based on tags within the OpenStreetMap data as follows:

  • max_speed: If a maxspeed tag is available from OSM, that speed is used as the routing speed and the speed_limit is set to that value.
  • highway: If there is no maxspeed tag, then speed is based on the OSM highway tag. There are a default set of speeds for each highway tag. Note that future work involves implementing country-specific default speeds for highway tags.
  • road density: The road density (the length of drivable roads in kilometers per square kilometer) at each node in the routing graph is estimated during Valhalla data import. The road density is used to determine if a road is in a rural or urban area. Roads in urban areas have their speed reduced if there is no maxspeed tag. In the future, this method may be replaced with a more accurate measure of rural versus urban regions, but density produces adequate results for now.

The speed_type attribute defines whether the assigned routing speed is from a speed limit or based on the highway tag.