Which speed measure is typically smaller on a congested roadway?

Which speed measure is typically smaller on a congested roadway?

• A. Space-Mean Speed
• B. Time-Mean Speed
• C. Both are the same
• D. None of the above

Answer: (A)

Space-mean speed and time-mean speed measure different ways of averaging speeds in a traffic stream. Space-mean speed averages the speeds of all vehicles that cross a section and corresponds to the harmonic mean of those speeds. Time-mean speed averages the speeds of vehicles observed over a time interval and is the arithmetic mean. The harmonic mean is always less than or equal to the arithmetic mean when the values vary, and congestion creates a wide mix of slow and fast speeds. This makes the space-mean speed smaller because it weights slower speeds more heavily in the averaging over the section duration. For example, with speeds of 10 mph and 40 mph, the time-mean speed is (10+40)/2 = 25 mph, while the space-mean speed is 2 / (1/10 + 1/40) = 16 mph. In congested conditions, many vehicles move slowly, so the space-mean speed remains notably smaller than the time-mean speed.

Space-mean speed and time-mean speed measure different ways of averaging speeds in a traffic stream. Space-mean speed averages the speeds of all vehicles that cross a section and corresponds to the harmonic mean of those speeds. Time-mean speed averages the speeds of vehicles observed over a time interval and is the arithmetic mean.

The harmonic mean is always less than or equal to the arithmetic mean when the values vary, and congestion creates a wide mix of slow and fast speeds. This makes the space-mean speed smaller because it weights slower speeds more heavily in the averaging over the section duration. For example, with speeds of 10 mph and 40 mph, the time-mean speed is (10+40)/2 = 25 mph, while the space-mean speed is 2 / (1/10 + 1/40) = 16 mph. In congested conditions, many vehicles move slowly, so the space-mean speed remains notably smaller than the time-mean speed.