Of course, GPS position accuracy doesn't change in a cul-de-sac.

But if the sampling rate is too low, the accuracy of the "distance traveled" and "speed" calculations will be affected by this "high frequency" deviation into and out of the cul-de-sac.

Let's say you want an accurate measure of "distance traveled" and "speed", based solely on GPS measurements.

If, say, a route deviation into a cul-de-sac lasts 10 seconds, the GPS sampling rate must be 5 seconds or shorter. This 2x sampling rate requirement is based on digital signal processing theory called the Nyquist frequency.

Reduce the GPS sampling rate below the Nyquist frequency, and the distance measurement of routes with or without the cul-de-sac deviation will be the same (they will be aliases of each other).