Its not a burden of proof. It's rather a proximate cause. Which given a natural unbroken chain of events the loss could have been prevented.
So the legal office does not determine proof but, rather determins if the U.S. Government has any liability and failed to take reasonable and prudent measures to prevent the loss. In your case I think its possible. DPW should have repaired the grate.
But, also when you use a lowered suspension or lower performance air dams (CF Spliters) you assume risk. Read your policy but, the insurance company may deny your claim based on the fact a lower carbon fiber splitter is not a normal part of the car. Good news is they dont often deny a simple claim.
For example, if you sit in the 5th row back along the 3rd base line you should expect to see some foul balls come your way and if you get hit. The pitcher, hitter or club owner is not considered at fault for your black eye.