Your efficiency calculation is wrong. It isn't 25% of 300Wh, 300Wh is the
output. The input is 400Wh, with 25% loss or 100Wh. Not 75Wh as you have. And you have a bunch of other mistakes too.
Along the length of the road, that is 21kWh. At 75% efficiency, that is 7kWh into the road surface per car. Roughly 2.4GWh of energy ($200k worth of electricity) per day lost as heat into the roadbed in total. Assuming 4 lanes and a 3.7m lane width, route 60 is 1.7 million square meters of asphalt and that is 1.4kWh per square meter. (8x more than your number.) Still comparable to solar irradiation though. Asphalt might get a little soft on a busy summer day when everyone is also running their AC but it wouldn't be a molten puddle. If the chargers are only embedded in a narrow strip down the middle of the lane, that section might get very soft.
So at 75% I guess it is practical. But 75% is going to be extremely hard. And the first place this should be installed is Route 1, because it would be a highway-sized snow-melter that also happens to power vehicles.
Now ask yourself why we don't have defrosted highways. Installing roadbed chargers is going to be even more difficult.