It's relatively easy to quickly move 100K people in a city of 10M. But it's impossible to quickly move 100K in a city of 100K. It's all about proportions. There's not enough transportation infrastructure anywhere to get 100% of the population out.
In the US, building of new roads has not kept up with population increases (in part because there's no place to put new roads). Even the smallest towns tend to have delays and grid lock at rush hour, and that's only part of the population. The speed of the fires also requires a greater evacuation speed. A bicycle would probably be faster than a car given the capacity of the roads.
