The University of Michigan's Mcity Test Facility has released the first open-source digital twin of its test track, allowing researchers worldwide to simulate and test connected and autonomous vehicle (CAV) technologies. This tool offers a safer, faster, and cost-effective alternative to real-world testing, integrating with TeraSim, Mcity's open-source traffic simulator, to create dynamic testing scenarios. The Mcity digital twin is a significant milestone in mobility systems testing, including autonomous driving, and aims to accelerate the progress of autonomous vehicles by enabling extensive simulations before real-world deployment.