help needed on stat , i m new

I have a database:
Interstate ADT(avg daily traffic) IRL 1yrRL 2ndyrRL...5thyrRL
I-95 15000-25000 800 750 650 340

I-95 is a highway where average daily traffic is from 15000-25000. The roadmarking or roadsign that were put initially to guide the car/driver at nighttime had IRL(initial retro reflective number) of 800 and the material was polyurea. It can be thermo or paint also. Then an instrument is used and the reading of the roadmarking taken a year later and found 750 and then the later years 650,340...and son on may be upto 7th year.

I developed an exponential degradation model from the database but does not look satisfactory to my BOSS.

Now my goal is to find the cost effectiveness of these materials.

Is there any otherway you can suggest or advice me..

Thank you once again.

Regards
Sharif