Answer:

The correct answer is option C) Standard deviation.

Explanation:

The average difference between observed speeds and time mean speed is represented by the standard deviation. Here's a detailed explanation of why the standard deviation is the appropriate measure for this scenario:

Observed Speeds:
- The observed speeds refer to the actual speeds recorded during a specific time period or at specific locations.
- These observed speeds can vary from vehicle to vehicle and may not necessarily match the average speed.

Time Mean Speed:
- The time mean speed is the average speed of all vehicles observed during a specific time period or at specific locations.
- It is calculated by summing up all observed speeds and dividing by the total number of observations.

Average Difference:
- The average difference between observed speeds and time mean speed measures how much the individual speeds deviate from the average speed.
- It provides an understanding of the dispersion or spread of the observed speeds around the mean.

Standard Deviation:
- The standard deviation is a measure of the dispersion or spread of a dataset.
- It quantifies the average amount by which data points deviate from the mean.
- In this case, the observed speeds are the data points, and the time mean speed is the mean.
- The standard deviation will give us a numerical value that represents the average difference between the observed speeds and the mean speed.

Other Options:
- Option A) 85th percentile speed: The 85th percentile speed refers to the speed below which 85% of observed speeds fall. It does not directly represent the average difference between observed speeds and the mean.
- Option B) Median: The median is the middle value in a dataset when arranged in ascending or descending order. It does not directly represent the average difference between observed speeds and the mean.
- Option D) Pace: Pace refers to the time taken to cover a unit distance. It does not directly represent the average difference between observed speeds and the mean.

Therefore, the correct answer is option C) Standard deviation, as it is the appropriate measure to represent the average difference between observed speeds and the time mean speed.