Like I said: This is a great idea for urban/suburban driving dominated by stop-and-go. The regenerative braking recovers the energy lost stopping at a light.
For road use: Useless. Let me give you an extreme example.
Your F-350 is pulling a 10,000 lb
trailer full of lumber. Heavy but not aerodynamically draggy. You are on I-64 southeast of Charleston WV. I-64 crosses the New River at Sandstone, WV, and there is a long, steep grade on either side of the bridge. Both are seven miles long at about 7% grade, if I remember correctly.
You top the hill at one side bopping along at 65 and start down. Pretty soon gravity has you divebombing down that grade like a Dauntless at Midway. First thing you go to your generative brake. Even if the batteries or other energy storage were dead flat empty, you will fill them up within a half-mile or so and you have to go to your normal brakes. If you are like most people, you blast over that bridge doing 85 anyway. Pretty soon, though gravity starts doing its thing again and you go to the battery to boost you up the hill. Even if you had charged up to full batteries, they are dead flat in a mile or so, and you go back to the engine to get you up the other side.
You simply cannot carry enough batteries or air tanks to store that much energy. So why bother, other than urban driving?
This is a classic scenario for straight electric railroads. A 10,000 ton train climbing or descending a hill involves stupendous amounts of energy. The dispatcher would hold a train at the top of the grade until another started up then release the uphill train to go down. The train going downhill would go to regenerative braking (this stuff has been around for more than a century) but rather than charging batteries, he discharges the power back into the catenary and the train going uphill uses that power to climb the hill. The second train can climb the hill using 15% of the energy the first one used to climb the hill in the first place. Some open-pit mine use a similar system for their diesel-electric haul trucks. Old tech. Works reliably, but does not use storage but dynamic use of regenerative power.
The stop-and-go pattern of urban driving does not generate or use so much energy that a battery is out of the question. Line-haul is another kettle of fish entirely.