If you're looking for a traffic data web service, you probably have not been able to find one for free. Now mapping pioneer MapQuest is beta testing a service it says provides "real time traffic information related to incidents, markets and flow."
MapQuest (our MapQuest API profile) keeps plugging away at developer services, an area where it has been incredibly behind Google, Yahoo and Microsoft. The AOL-owned company, which has also seen its consumer mapping lead slip, has released several services that its competitors don't have. Last summer, MapQuest became the first to provide direct data access to driving directions.
The new traffic service is in a similar vein. MapQuest returns either XML or JSON of construction and other incidents within a geographic area. What you do with that data is then up to you--the terms don't even seem to stop you from putting it on a competitor's map.
Traffic data is valuable. Companies, such as INRIX, are paid a pretty penny to provide it to partners. And it's worth it, because right now it's still difficult to collect. New companies, such as Waze, are attempting to crowdsource traffic data, so expect the landscape to change dramatically in the near future.
In the meantime, MapQuest is still the only one providing this data free of charge, currently as a beta product. Check out its developer guide, which includes a nice sandbox to test out a few calls to the new API.