30 MPH traffic would require a prohibitive number of small cells for carriers to do it themselves. Wi-Fi First is not a complete substitute for LTE towers. There will always be spots not covered because small cells, especially in higher frequencies, have very short range. Below, the CEO of American Tower puts forth his opinion why highways and moving vehicles are particularly difficult to cover. James Taiclet via Seeking Alpha.
Taicet. The handoff requirement from places where our towers serve people, which are often around highways and other transportation corridors, suburban or rural, you've got people traveling 30 miles an hour to 60 miles an hour. You can't really have sufficient handoff capability over a very large stretch of multi-mile roadway to economically provide those handoffs.
Taicet, thinking of carrier small cells, went on to say the economics become dubious with densities of less than 10,000 people per square mile. Wi-Fi First changes that, of course.