Answered By: Ken Winter
Last Updated: Apr 10, 2017     Views: 6

The Impact factor (IF) or Journal impact factor (JIF) of a scholarly journal has been used since 1975 as a metric for relative "importance" of journals and other publications to a field of research. 

Technically, the IF is a measure reflecting the yearly average number of citations to recent articles published in a specific journal, so each journal as an IF and also a 5-year IF.

Calculation: In any given year, the IF of a journal is the number of citations received in that year by articles published in that journal during the two preceding years, divided by the total number of articles published in that journal during the two preceding years

Journals with a higher IF are typically considered "more important" or prestigious (especially for authors seeking publication) than those with lower IF, however, impact factors and the broader concept of "citation analysis" is subject to field-dependent factors which may invalidate comparisons not only across disciplines but even within different research areas of a single discipline. In short, a "good" IF for one topic may be a poor IF for another. 

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.