Life is defined by base sequences of DNA, a polymer that carries information on how to make proteins, which in turn serve as building blocks and functional units within living cells. As a first step toward making proteins, DNA is read and copied by processive molecular motors, called RNA polymerases (RNAPs). Multiple RNAPs travel on the genomic DNA at the same time, like cars on a highway. While the dynamics of this molecular traffic is important in understanding gene expression, we lack a clear understanding of how the traffic is formed and regulated inside the cell. Experimental results have suggested that RNAPs can exhibit collective group dynamics by exploiting dynamic changes in DNA topology. In this talk, I will describe our current understanding of RNAP traffic on the genome and will explain how biophysics can help solve the remaining questions and challenges.