Bidirectional motion estimation (ME) significantly enhances video coding efficiency, whereas its huge complexity is also a critical problem for implementation. This paper presents alternating asymmetric search range assignment (AASRA) schemes to reduce the complexity by switching the use of a large and a small search ranges (SR.L and SR.S). A temporal AASRA (T-AASRA) scheme performs search range switching between past and future reference directions. A temporal-spatial AASRA (TS-AASRA) scheme performs more aggressive switching on the two dimensions of reference direction and MB/CTB index. T-AASRA and TS-AASRA achieve 43.5% and 65.2% complexity reduction, respectively, with small coding efficiency drop. Even after removing the factor of coding efficiency drop, the two schemes still show 38.4% and 54.7% equivalent complexity reduction. AASRA can also be combined with existing fast algorithms for further complexity saving, which has been demonstrated on hierarchical ME and dynamic search range selection algorithms. (C) 2014 Elsevier Inc. All rights reserved.