30.3. Speed

Speed is considered as a quality measurement of travel as the drivers and passengers will be concerned more about the speed of the journey than the design aspects of the traffic. It is defined as the rate of motion in distance per unit of time. Mathematically speed or velocity v is given by, v = d/t where, v is the speed of the vehicle in m/s, d is distance travelled in m in time t seconds.

Speed is a critical factor in traffic engineering because it directly impacts how drivers and passengers perceive their journey. It is defined as the distance traveled by a vehicle divided by the time taken to travel that distance. This can be expressed mathematically with the formula v = d/t, where:

• v represents speed (in meters per second, m/s),
• d is the distance covered (in meters),
• t is the time taken (in seconds). This formula helps traffic engineers assess how fast vehicles are moving, enabling them to design roads better.

Imagine you're driving on a highway and you keep track of how far you travel and how long it takes you. If you drive 120 meters in 4 seconds, your speed would be 30 meters per second. Just like when you time your run on a track, knowing your speed helps you understand and improve your journey.

Speed of different vehicles will vary with respect to time and space. To represent these variations, several types of speed can be defined. Important among them are spot speed, running speed, journey speed, time mean speed and space mean speed.

Different vehicles exhibit various speeds depending on the conditions of the road and traffic. To categorize these variations, traffic engineers define several types of speed:

• Spot Speed: The speed of a vehicle at a specific moment or location.
• Running Speed: The average speed of a vehicle only while it is moving and not when it's stopped.
• Journey Speed: The average speed of a vehicle during an entire trip, including stops.
• Time Mean Speed: The average speed of all vehicles passing a certain point over a specified time period.
• Space Mean Speed: The average speed of all vehicles in a specific length of road over time. These distinctions are essential for accurate traffic analysis and road design.

Think of a delivery driver. The spot speed indicates how fast they’re going when checked at a specific moment, whereas the running speed is how fast they travel while the vehicle is actually moving, excluding any time spent at traffic lights or in traffic jams. The journey speed gives an overall view of how efficient their deliveries are, including time spent unloading. In a typical time of commuting, you might find your time mean speed is higher during morning rush hours when more cars are out, but the space mean speed might indicate slower traffic due to congested conditions down the street.

Spot speed is the instantaneous speed of a vehicle at a specified location. Spot speed can be used to design the geometry of road like horizontal and vertical curves, super elevation etc. Location and size of signs, design of signals, safe speed, and speed zone determination, require the spot speed data.

Spot speed reflects how fast a vehicle is traveling at a specific point on the road at any given moment. Engineers collect data on spot speed to make various decisions regarding road design, such as creating safe curves, adjusting elevation to manage water drainage, positioning signage effectively, and determining speed limits. Knowing the spot speed helps officials assess if roads are being used safely and effectively.

Imagine you are measuring how fast cars are traveling as they pass a certain traffic light on your way to school. By knowing the spot speed of vehicles at that point, city planners can decide how to improve safety in that area—perhaps by adding a speed limit sign or modifying the traffic signal to accommodate rush hours.

Running speed is the average speed maintained over a particular course while the vehicle is moving and is found by dividing the length of the course by the time duration the vehicle was in motion.

Running speed is calculated when a vehicle is in motion but does not account for any stops that occur during a trip. To calculate it, you take the total distance traveled and divide it by the amount of time spent moving, giving a clear picture of how efficiently the vehicle operates on the road without interruptions. This measure is vital for understanding how road conditions might affect travel time in real scenarios.

Consider a cyclist who rides 2 kilometers on an open road in 5 minutes without stopping. Their running speed would be 2 km divided by 5 minutes, giving you a better insight into how fast they're going when the road is clear compared to when they might have to stop at traffic signals.

Journey speed is the effective speed of the vehicle on a journey between two points and is the distance between the two points divided by the total time taken for the vehicle to complete the journey including any stopped time.

Journey speed gives an overall representation of how fast a vehicle travels between two locations, factoring in all stops, such as traffic lights and delays. To determine journey speed, measure the total distance between the start and end points and then divide that distance by the total time (moving plus stopped time). This speed often varies significantly from running speed, especially in busy traffic situations.

Imagine you are driving from your house to a friend's house across town. The trip takes you 30 minutes in total, which includes 10 minutes waiting at traffic lights. If the distance is 10 kilometers, your journey speed would be 10 km ÷ 30 minutes. This helps you understand that despite how fast you could go on the straight stretches, real-world driving involves a lot of stopping.

Time mean speed is defined as the average speed of all the vehicles passing a point on a highway over some specified time period. 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. 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 is calculated based on the speed of vehicles as they pass a certain point on a road within a set timeframe. In contrast, space mean speed takes into account the speed of vehicles within a specific section of road over time. These two measurements typically yield different results, as they assess speed from different perspectives—one from the standpoint of time at a specific point and the other based on space across a section of road.

Think of a concert venue where cars are arriving at different times. If you measure how fast cars are arriving at the gate, you'll find the time mean speed for that specific moment. However, if you look at how vehicles are distributed across the parking lot at that moment, that gives you the space mean speed. This distinction helps manage parking and ensures everyone gets in smoothly.

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