Spain has the largest High-Speed Rail (HSR) network in Europe, and in per capita terms, the largest in the World. It has been largely funded by the European Commission under the premise that this type of infrastructure provides a cleaner mode of transport than other modes, as the cars, and provides a better solution for reaching their commitments in terms of enviromental policy and sustainable transport. However, for this policy to work, there should be an actual inter-modal substitution from more polluting mode of transportation to the HSR. This paper takes a comprehensive approach and analyzes the causal impact of new HSR routes openings on motorway transport in Spain in a traditional quasi-experimental setting (e.g., Generalized Diff-in-Diff) and using the new literature on staggered difference in difference (e.g. Callaway and Sant’Anna (2021)). Unlike previous studies, our paper analyzes the whole HSR network and not just one single HSR route. We matched the data on light-vehicles traffic for the whole State’s highways system with georeferenced location on origin-destination routes using Google Maps API to estimate the ATT effect of the entrance of the HSR lines on motorway’s traffic. We found that the HSR has not generated a significant level of modal split, but only for some of the HSR segments. Only on these segments, the effect is estimated in a reduction of 2 thousand cars on an annual average daily traffic basis. However, this effect is individually significant only until 10 years since the entry of the HSR. These results have serious implications in terms of the Transport policy in Spain, as well as in the use of tax-payer money on Transport infrastructure and Government planning.