30.3.4. Time mean speed and space mean speed

Time mean speed is defined as the average speed of all the vehicles passing a point on a highway over some specified time period.

Time mean speed calculates the average speed at which vehicles pass a specific point on a road over a defined period. For example, if we measure the speed of all vehicles that pass a specific point from 1 PM to 2 PM, we can determine the time mean speed by averaging their speeds during that hour. This provides insights into how quickly the traffic is moving at that location during that specific time.

Think of a speed limit sign on a highway that tells you the average speed of cars passing it during rush hour. If over an hour, cars passed by at various speeds, the time mean speed gives you a single value representing that average speed, which helps in understanding traffic flow.

Space mean speed is defined as the average speed of all the vehicles occupying a given section of a highway over some specified time period.

Space mean speed focuses on the average speed of vehicles occupying an entire stretch of road at a specific instant. For example, if we look at every vehicle on a 1-kilometer stretch of road at 2 PM and measure their speeds, we can compute the space mean speed. This measure considers how vehicles are distributed along the road, which might reveal different traffic dynamics compared to time mean speed.

Imagine a crowded highway where vehicles are spread out over a certain distance. Space mean speed is like asking, 'What is the average speed of all vehicles along this stretch at this moment?' It's helpful for understanding congestion levels over the entire stretch of road, much like calculating the average height of items in a storage area.

Both mean speeds will always be different from each other except in the unlikely event that all vehicles are travelling at the same speed.

Time mean speed and space mean speed usually result in different values because they measure speed in distinct ways. Time mean speed is focused on a specific point over time, while space mean speed takes the entire stretch of the road into account. This difference becomes particularly noticeable in scenarios of varying traffic speeds and vehicle distribution, such as in congested or free-flowing traffic conditions.

Consider a racetrack with cars speeding past a checkpoint (time mean speed) versus a survey of all cars on the track at the same moment (space mean speed). If every car is going full throttle but some cars start later, their speeds at that checkpoint may average out differently compared to if we measured every car at a specific time, resulting in two different speed measurements.

Time mean speed is a point measurement while space mean speed is a measure relating to the length of highway or lane.

Knowing the difference in application is crucial for traffic planning and management. Time mean speed helps in real-time traffic monitoring, identifying how well traffic is flowing at specific points, which is vital for signal timings and congestion management. Conversely, space mean speed informs roadway design and capacity planning, helping engineers understand how effectively space is being utilized at any given time across a length of highway.

Imagine you're a city planner. If you're investigating how a traffic signal affects vehicles at a particular intersection, you'd use time mean speed. But if you're evaluating overall lane capacity on a highway to decide if it needs an upgrade, you'd consider space mean speed to see how the entire stretch performs under various conditions.

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