A translucent thin film that could change colors by changing its molecular arrangement given electron or photon movement thru the film needs to be invented.
A sensor that can send signals to the smart windshield also needs to be invented.
A GPS that could transmit information needs to become affordable.
A set of wavelengths that will drive these transmissions needs to be determined to avoid malfunction due to interference.
COST, the I-Streets sensors and electronics that will be positioned in each car should not be expensive and on the first phase, people can acquire those and placed them in their current cars, new cars will all be sold with the new technology, for the second phase it will be necessary to replace all cars or make sure all have it so this will be a big change (since traffic lights will be removed) and will require serious advertisement and funds to provide to all drivers the needed sensors (just as to what happened when TVs went from analog to digital).