I'd like to find out the relation between the speed and density of traffic flow, and now I need some real data, but I don't know how to find it. Could you please tell me?
Try the Highway Performance Monitoring System (HPMS) from the US Federal Highway Administration. Esri shapefiles available here: http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_atlas_database/2013/zip/hpms.zip. Full National Transportation Atlas Database (NTAD) available here: http://www.rita.dot.gov/bts/sites/rita.dot.gov.bts/files/publications/national_transportation_atlas_database/index.html
I hope this helps. If you are looking for counts I think that can be found at some city sites. If you are looking for counts and speed you may need to buy the data.
So this might be a start. Chicago has an open data project. Which includes Average Daily Traffic Counts.
Average Daily Traffic refers to the number of vehicles traveling through a particular point on the city streets in a 24-hour period. Average Daily Traffic (ADT) counts are analogous to a census count of vehicles on city streets. These counts provide a close approximation to the actual number of vehicles passing through a given location on an average weekday.
This site below sell traffic data.
I will have to make a 2nd post this I need a high reputation to post more than one link:(
Also, the Open NY Data project has real time speed data. I'm not sure if it has counts as well.
Does this type of data work for you?
I passed the question onto a librarian from a state dot (department of transportation). His response was :
Transportation data of that type would need to requested from a state or city transportation department specifically. The requestor should start with the state dot.
Transportation data is not yet open in many places. That is some thing we are working on.
I had talked to him earlier about the opening up of transportation data in general, and he had mentioned that each state had different rights that they tried to assert over their data. As many states have rules similar to the Freedom of Information Act, it's possible that you could go that route if you really needed to.