About purchase api for maxspeed and its usage limit

OpenStreetMap is a volunteer driven project that collects Geodata. We do not have data about every road in the world, just about those that have been added by our volunteers.

Only about 5% of all roads that are in OpenStreetMap world-wide carry maxspeed information. I don't know how that percentage looks like for the US specifically; I would assume that it is less than 5%.

If you want an interface that, for a given latitude and longitude, returns the street you're on with maxspeed information, you should install the Nominatim geocoding software on your own server and hire someone to modify its return data by including the maxspeed information. Or do that modification yourself, since everything is Open Source.