"The most fair would be for each person to bear their portion of the cost that they create. Since that isn't practical, a rough approximation is the next best thing. How much of the maintenance costs do they cause? As a rough approximation, it's proportional to the amount they use the resource (the road, in this case)."
Which is exactly what I'm proposing. The employer is depending on the road infrastructure to facilitate it's business function, namely, to get all the employees who generate 100% of it's fiscal gains to where it's needs them to be. Since it gets the majority of the financial gains it should pay the majority of the expense. Saying the employee is the one using the road is equivalent to charging this tax to the truck drivers a company employees. Want to test it? How about all the employees of HP not utilize any public infrastructure for their commute one month without detaining staff and see if it impacts their revenues. If not being able to use buses, subways, sidewalks, roads, traffic signals, or depend on the protection of police has no impact on HP's business you win if it does then the commute is in fact part of HP's business function.
"The fact that one person, for whatever reason, has accumulated more dollars is not directly relevant at all. It's just like when you go to the grocery store to buy a loaf of bread - the price is based on the cost of ingredients, the preparation costs, shipping, wages for workers, some portion of the store's overhead, some portion for profits. Nowhere in that equation is how much money you make, how hard you work, etc."
It is completely relevant. Bread is not public infrastructure. But it does cost in terms of public infrastructure to make, it causes wear and tear for employees to commute, it costs on the subsidized power grid, it costs to enforce the regulations to make a bread shop produce safe product, it costs to provide police to protect it, it costs federal funds out the wazoo to support the banking structure and security, healthcare costs to keep staff healthy and able to produce the bread, drive the trucks, repair equipment, education for the same, etc. The things you list are actually only a minor factor in the price of bread or anything else, that is the minimum cost, the actual cost is whatever the consumer can be gotten to pay. However all these expenses are present for every loaf of bread, and if a loaf of bread is $1 and I have $10 dollars in my pocket I have the public resources required to produce 10 loaves of bread sitting in my pocket. If you have $100, your $100 represents 10x as many public resources.