Difficulty Renting- Use Back to Back Double-Pane Windows For Loud Road Noise?

[Burt L.]

Title says most of it- I have lots of interest but since the property faces a "main arterial route", which translates to road noise. The two units at the front of the property are hard to rent while the rear ones are not. Having an interest rate obtained after covid, it takes full occupancy  to cover the debt on the property. 

Having tried the smaller things like accoustical caulking and thicker door weatherstripping, I need to think of further methods. The windows are double pane and attach to the outer edge of the windowsill.  In looking at my own home which has single pane windows, I see that there are storm windows which attach to the inner edge of the windowsill. It seems I could buy another set of double-pane windows and attach them to the inner-edge of the windowsill. The windows in this area are commonly bypass slider type windows and the openings would line up, allowing them to be opened, just as my own storm and outer windows line up. The construction type is cinderblock with a brick exterior on the rental. I don't' think the cinderblocks are filled but would be hard to get to under the eaves and ceiling. 

There would be about a 3 inch gap between the inner and outer double-pane vinyl slider windows. It would be hard to clean the stationary section on the large living room window, which is 8 ft by 5 ft and has a slider on each end of about 2 ft in width. 

There is a company called "Indows" that will make a single pane that attaches to the inside with levers for removability but understand they are $500 to $1000 per window and I can get double-pane  vinyl sliders at $3-4 hundred each and probably $500 for the large one. It will be a long 3 weeks wait to have them made. They could also be spares if one gets broken, though the outer windows don't sit on the windowsill and are about 2 inches taller and wider. 

This is a low-frequency noise issue. There is a fence in front limited to 4 ft with 50% coverage as that is all the city allows. I've also thought of trees in front but that takes a lengthy time to grow to usefulness. 

Am I missing somethig here? This is an expensive problem with every turn-over on the front units and am sitting in an empty unit writing this now. Thank you for reading thru.