Journal Impact Factor is a measure of the frequency with which the "average article" in a journal has been cited in a particular year. The impact factor will help you evaluate a journal's relative importance, especially when you compare it to others in the same field. Not every journal will have an impact factor but there are other ways to measure impact to a field.
Impact Factor of a journal can be looked up through the Journal Citation Reports (JCR) database, part of the Web of Science subscription available at the University of Virginia. For help contact the VDOT Research Library.
The latest edition of JCR is the default option and is a year behind the current calendar year. You can search for a specific journal or subject area. Since impact factors vary widely by discipline, it is helpful to get a baseline for your discipline by searching for subject area first. Impact factor data is only available for journals indexed by Web of Science and consulting Journal Citation Reports' master journal list can confirm if a publication is indexed by JCR, which is a required for obtaining an IF.
Web of Science will provide the Journal Impact Factor for the current year, the 5 year impact factor, the immediacy index, the journal half-life, and Eigenfactor scores.
In recent years fraudulent impact factors have been produced by companies not affiliated with Journal Citation Reports, and have been used by "predatory publishers" to lure unsuspecting authors into submitting papers for publication, sometimes with a fee.
If you are curious about how the "Impact Factor" was created and how it can be used, consider reading: The Agony and the Ecstasy — The History and the Meaning of the Journal Impact Factor by the creator of the IF, Dr. Eugene Garfield.
Some recent Impact Factors for well-known journals:
Transportation Research Record (TRB)
Impact Factor (2015): 0.522
5-Year Impact Factor: 0.778
Transportation Research Part A: Policy & Practice (Elsevier)
Impact Factor (2015): 1.994
5-Year Impact Factor: 2.998
Transportation Research Part B: Methodological (Elsevier)
Impact Factor (2015): 3.769
5-Year Impact Factor: 4.833
Transportation Research Part C: Emerging Technologies (Elsevier)
Impact Factor (2015): 3.075
5-Year Impact Factor: 3.631
Transportation Research Part D: Transport and Environment (Elsevier)
Impact Factor (2015): 1.864
5-Year Impact Factor: 2.366
Transportation Research Part E: Logistics and Transportation Review (Elsevier)
Impact Factor (2015): 2.279
5-Year Impact Factor: 3.319
Transportation Research Part F: Traffic Psychology and Behaviour (Elsevier)
Impact Factor (2015): 1.44
5-Year Impact Factor: 2.028
Journal of Transportation Engineering (ASCE)
Impact Factor (2015): 0.801
5-Year Impact Factor: 1.039
Transportation (Springer)
Impact Factor (2015): 1.545
5-Year Impact Factor: 2.195
In recent years the old-school metric of Impact Factor has been challenged by non-traditional or "alternative" metrics, sometimes referred to as Altmetrics. Altmetrics did not originally focus on citation "counts" like Impact Factor, but instead recorded other aspects of the impact of a work, such as how many data and knowledge bases refer to it, article views, downloads, or mentions in social and news media.
Proponents of Altmetrics point out that many of the metrics factored in show influence or "engagement," rather than impact on the progress of science compared to more traditional citation impact metrics, such as Impact Factor.
