Highway classification based on traffic count

Is it appropriate to adjust the highway tag based on traffic counts? There is a highway=secondary that has 2.5x the traffic volume of a nearby highway=primary, and I would like to correct this.

I would either be using my own traffic counts or the city’s, if appropriately licensed.

You can use car counts but only to aid your judgement as mapmaker, not as hard thresholds.

Deciding on a precise hierarchy level for every road is ultimately subjective and depends on the network as a whole. It can not be judged individually for each road. There is no one correct hierarchy level, but an useful hierarchy level. Some things that are wanted from hierarchy is that it simplifies navigation. To manually route from place A to place B, one goes up in the hierarchy, then down when approaching the destination. It is expected that a given hierarchy level continues until looping or ending in an equal or higher level. Using only car counts would probably not met these desired properties.

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I agree that no single metric (measurable quantity), like car counts, can capture the many nuances of how roads are classified. In UK, where our “primary, secondary, tertiary…” tags came from, these definitions / classifications are much more strict, even legally defined. But around the world (in other, non-UK countries), as mentioned, they have become more subjective. These subjective inputs might, even do, include car counts to determine a highway’s classification, but many such inputs are included in the ultimate determination of not only the road itself, but “how it fits into its network / hierarchy.”

So, no, “just” (or “simply” or “only”) traffic count wouldn’t be “enough” to classify any given highway. In non-UK countries, that is.


in the Wiki it is explicitly stated, that the factual importance of a road overrides for example the official classification.


However, as this has an subjective aspect, changing main roads may lead to conflicts between local mappers. But if you are sure, you may change the highway classification.

note that there could be road of local importance within city with higher car volume that road forming national road network (note: actual road network, it is not about being officially classified in this way)

in such case road with lower traffic may have higher road classification

this case is extremely rare, but may be happening

Highway classification differs by country and there’s no universal answer to this. You’d need to say which country you’re in for a helpful answer.


United States

Thank you all for your responses, particularly the clarification that the classification is subjective and based on how it fits into the road network. Looking at the wider network, I suspect the road that is currently tagged highway=primary should be downgraded to highway=secondary. I’ll mull it over and perhaps ask another local mapper for their opinion.


This is exactly how I did it in my place of residence.

  • Think about it: how is the “official” road network designated, how are the connections signposted, how would I drive from A to B as a “stranger” and how would I drive from A to B as a local with local knowledge. After that, the importance of a road can usually be assessed quite well.
  • Ask other local mappers for their opinion. At that time there were not many of us (only one, to be exact), but where we came to the same conclusion (especially secondary vs. tertiary vs. residential), we also changed this in deviation from the “official” classifications. This road network has remained unchanged even after many years.

The U.S. community is in the process of overhauling the rubric for highway classification in each state. The current national guidelines prioritize connectivity, which can correlate to traffic volume along long-distance highways. But at that scale, you probably don’t need traffic counts to know that a particular U.S. Route is more important than the parallel state routes. Some mappers use routers to gut-check their assumptions about the main route between major cities, though there’s a risk of circular logic, since routers also factor in highway classification.

As you’ve probably discovered, connectivity-based criteria tend to break down in urban street grids. Some of the state-specific guidelines are attempting to address this issue, but generally it’s still a matter of experimentation. Traffic counts are a useful input, but locals often do a better job of interpreting them. For example, in Cincinnati, we had some back and forth along the riverfront because a normally quiet street that serves two stadiums gets a ton of traffic on game day, skewing the AADT metric. Another street gets a fair amount of traffic solely because it’s a truck route, but the general motoring public doesn’t really care about it.

Valhalla can be set to “shortest distance”.

It can, but that isn’t particularly relevant for this use case. I meant that U.S.-based mappers have often used a router’s normal fastest-time driving profile to justify their claim that some highway is the most useful connection between two cities. This judgment is based on time rather than distance. I think it’s OK to consult a router as a gut-check – as long as it’s an OSM-based router! – but also important to take the routes with a grain of salt, especially in cases where we suspect that roads are already improperly classified.

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Of course. That goes to what I was saying earlier here: There is no measure that substitutes judgement when assigning road hierarchy.

Fortunately, I have not had problems with this with local mappers in Mexico.