Partial- and conditional-automated driving systems (ADS) can not only assist drivers with their driving tasks but also significantly reduce the driving-related burden. Yet still, when the AVS is engaged, the human driver still plays a critical role such as monitoring the driving environment and performing certain driving tasks when called upon by the ADS. There exists ample evidence in the literature on simulation experiments and real-world that point to the difficulty of human drivers to maintain the requisite situational awareness to safely take over the vehicle when needed. This is often due to the nature of the non-driving related tasks in which they typically engage, low vigilance, and excessive trust in ADS capabilities. There exists a need to assist drivers to maintain a certain minimal level of situational awareness, to promote smooth and safe transition of the vehicle from the automated driving system to manual control where necessary. This study developed inputs for an in-vehicle situational awareness enhancing system (SAES) and a prototype SAES, to facilitate AV-to-manual takeover in partially and conditionally automated vehicles. The study is predicated on the notion that appropriate inputs to SAES can help it effectively direct drivers’ attention to prospective AV-to-manual transition thereby increasing takeover quality and reducing takeover time. This study also synthesized evidence from past studies on the benefits of enhanced situational awareness on takeover performance in partial- and conditional-automation driving environments. Finally, a SAES case study involving comfortable headways was carried out through a cab-driving simulation experiment. At a broader level, the study results can help guide the design of in-vehicle alerts intended to enhance situational awareness and the development of AV operator training manuals.