How to go about obtaining data I am a researcher working on traffic flow prediction. For my project I would need to obtain historical traffic flow raw data. Google Maps has a traffic flow overlay which is pretty much the kind of data I need. However, Google doesn't seem to provide access to historical data. There are several other companies that provide analytics based on traffic data to their clients but raw data is never provided. How would one go about obtaining this kind of data? 

Edit: the kind of data I need would be spatio-temporal. Take a bunch of N sensor locations on an inner city road network and give me the driving speed of the cars passing through each one of them at T equally spaced time points during a day. So we would have N*T data points per day. The denser the locations the better. 
If you look at Google Maps traffic overlay you can see traffic flow speeds for almost every small street. I think they take this data from GPS in driving cars as well as stationary sensors. Google doesn't provide historic data but maybe one could record this from the website directly?
You should try contacting cities about getting the sensor data from their Traffic Controller Systems. These usually have several detectors per lane at intersections (they also control the traffic lights so you get data on those as well). Some of these data are property of the companies that supplies these system, in which case you'll need to contact the company. These companies are generally not keen on sharing the data with commercial parties, but generally are interested in sharing it with educational parties as outreach.

I know one public dataset of this kind of data, the city of Utrecht in the Netherlands provides a sample of traffic density data. Unfortunately the metadata is mostly in Dutch.