Is there a site where USA road traffic historical data would be available? For example, whether the usual traffic jam on Bay Bridge in Maryland on Memorial day weekend was worse in 2012 vs. 2011?
Departments of Transportation across the United States collect data for the Highway Performance Monitoring System requirements. The collected data is conflated and provided as the HPMS shapefile by the National Transportation Atlas Database, click on the year, select polyline, and download the HPMS shapefile. The HPMS shapefile data is the Annual Average Daily Traffic for most major roadways, however, it is merely a single value and does not provide diurnal data.
As part of the collection process to develop the HPMS, Departments of Transportation collect traffic via a continuously collecting permanent monitor or a certain window of time monitor. If you go to Maryland's DOT website for traffic counts, they allude to continous traffic count locations, they have 82 continuous automatic traffic recorders. Download the shapefile, see if the ATR is at the location in question, then send an information request for the database containing all the ATR data by hour. This process can be done for all of the states, but different DOT's have different policies on sharing this data, especially if you are not a resident of the state.
In Florida, you can find these continuously monitored site with hourly counts. You can obtain samples of this data in Florida by going to the Florida Traffic Online site and looking for Red Dots, TTMS locations, and selecting continuous hourly counts. Each state is different on how they make this data available and how they go about counting it.
Nokia's Here Traffic, sold here by NavMart, provides 15 minute speed based probes of major roadways across the globe. Their historical data would be able to provide speed data to answer historical questions on how long it took to clear a bridge from congested speed to free flow speed.