A civil engineer wants to estimate the maximum number of cars that can safely travel on a particular road at a given speed. He assumes that each car is 19 feet long, travels at speed s, and follows the car in front of it at a safe distance for that speed. He finds that the number N of cars that can pass a given spot per minute is modeled by the function N(s) = \frac{86 s}{19 + 19 \left(\displaystyle\frac{s}{23}\right)^2} At what speed can the greatest number of cars travel safely on that road?

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