How slow does traffic need to be going on the highway in order to justify taking surface streets?

I’d say take the streets for a week. Each day record the time it takes you to take the streets. Find the average.

From there, figure out how far you’re actually going on the streets and how far you’re going on the highway. (use google maps…or a long tape measure)

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Time highway = Distance highway /Speed highway

Take the streets when :
Time streets < Time highway

So:
Time streets <Distance highway / Speed highway which yields

Take the streets when:
Speed highway < Distance highway / Time streets

once you find the distance and time it should give you something like ” speed < 50 mph” Once you know that equation, there’s your rule.