Hello, 

I think your approach is fair as you cover the most important steps in these kinds of market sizing: 
1. Start with Top-Down and slice the “market” into segments → Narrow down on the one you are interested in

2. Further break down your segment into different caregories: to go further you could've identified different needs for the categories of street you mentioned (e.g. high-way easy to paint, 1 layer, country road harder to paint, 2 layers)

3. Your assumption #4 is the right way to look at it - here as you don't know, selecting something that makes sense and is easy to calculate is the best approach (round numbers, easy to divide/multiply)

4. Very good to not stop at “the final number is X” but to include contingency / risks. You could also mention what additional data points you would want to validate to ensure that your approach is comprehensive. 

==> Remember that the goal of these exercises is not to get to the right number, but to see how you think and approach a problem in a few minutes to get to a sensible answer. 
 

Good job !
Happy to chat if you have questions, Cheers!