Older drivers tend to suffer more from aging related issues concerning hearing and vision, the reaction time, and cognitive functions. This research aims to help older drivers select proper gaps by using connected vehicles technologies and an in-vehicle advisory system. A study was conducted on 79 old drivers (60 or older), among them 75.9% were male and 24.1% were females. The study was conducted at two unsignalized intersections in the Stow and Tallmadge in Ohio, and it included both a simulation and a field test. The field data showed that a safe gap of 7.4 seconds at the Stow location can be used without causing speed impedance to the oncoming vehicles, compared with 6.5 seconds recommended by HCM. The in-vehicle safety advisory system provides warning through red flashing, beeping, and text messages on an LCD, if the upcoming gap is determined unsafe to use. When the system does not issue any warning message, it is the driver’s responsibility to decide if or not to take the available gap. The test involves two scenarios, one is when all vehicles are connected and the other is in a mixed traffic situation with both ordinary vehicles and connected vehicles. Different hardware designs and field test procedures were used to separate the two cases. The simulation runs used the field data to define modeling parameters. The measures of effectiveness (MOEs), such as average delay, queue length, and average waiting time, were obtained to assess the impact of running the advisory system. The field test results showed that 96% of the participants accept the system and agree that it can improve safety. Most of the participants claimed that they prefer a combination of beeping and red flashing rather than a single type of warning.