35.4. Types of Facilities

Most important classification of transportation facilities from the engineering perspective is based on the continuity of flow, that is uninterrupted flow and interrupted flow. Uninterrupted flow is the flow of traffic in which there is no obstructions to the movement of vehicles along the road. Freeway is one example for this type of facility. In a freeway, when a vehicle enters a freeway, there is no need for the vehicle to stop anywhere till it leaves the freeway.

Uninterrupted flow refers to a type of traffic flow where vehicles can continue moving without stopping due to traffic signals or other interruptions. This is commonly seen on freeways, where vehicles can enter and exit without stopping, allowing for smooth and continuous travel. An example of this is when a driver merges onto a freeway and can maintain speed without hitting any red lights or stop signs until they exit the freeway.

Think of uninterrupted flow like a river flowing freely without any dams or obstacles in the way. Just like how water can continue to move swiftly downstream, vehicles on a freeway can travel without any breaks, making the journey faster and more efficient.

There are three sections in a freeway - basic unit, weaving section and ramps (on/off). Vehicles will be entering the freeway through ramps. Ramps used for entering the freeway is called on-ramps and those used for exiting the freeway are called off-ramps. Freeways generally have 4, 6, or 8 lane alignments.

A freeway is divided into various sections to ensure smooth traffic flow. The basic unit represents the driving lane, while weaving sections allow vehicles to change lanes safely, typically near interchange areas. Ramps facilitate entry and exit. On-ramps allow vehicles to merge onto the freeway, while off-ramps let them leave the freeway. The design typically includes multiple lanes (4, 6, or 8) to accommodate high volumes of traffic.

Consider a freeway like a multi-lane highway where the on-ramps and off-ramps are similar to ramps in a skate park. Just as skaters can smoothly enter and exit the park without disrupting those already skating, vehicles can enter and exit the freeway seamlessly using ramps, maintaining the flow of traffic.

Interrupted flow refers to the condition when the traffic flow on the road is obstructed due to some reasons. This is experienced in signalized intersections, unsignalized intersections, arterials etc. At signalized intersections, there will be some kind of active control and the vehicle will have to stop or sometimes to reduce its speed and the flow of traffic is interrupted.

Interrupted flow occurs when obstacles such as traffic lights or intersections cause vehicles to stop or slow down. This is commonly experienced at traffic signals where vehicles must come to a halt before proceeding, leading to interruptions in the flow. This type of flow can greatly affect travel time and overall efficiency of the roadway.

Imagine an amusement park ride where the guests need to get off at each stop. If everyone needs to disembark before proceeding to the next part of the ride, it creates pauses in the experience—like how cars must stop at traffic signals, causing interruptions in their journey.

Thus the capacity is defined in terms of control delay ie sec/veh. Arterials are roads of long stretches with many intersections in between and obviously there will be interruption to the flow of traffic. Here, the capacity is expressed in terms of average travel speed.

The capacity of interrupted flow types of facilities is assessed using control delay (measured in seconds per vehicle), which indicates how long vehicles have to wait due to traffic signals or other interruptions. For arterials, which are long roads with several intersections, the average speed becomes the relevant metric for determining the capacity since frequent stops will impact how quickly vehicles can travel.

Think of a capacity measure for arterial roads like the time it takes to cook a meal that requires several steps. Each step (like stopping at an intersection) adds time to the overall cooking process. Similarly, each stop that cars encounter on an arterial road adds delay to their trip, impacting the average speed.

Some other facilities are facilities for pedestrians, bicycles, bus-transit, rail-transit etc. Example for pedestrian facility is a provision of subway exclusively for the use of pedestrians. Here, the capacity may be expressed in terms of number of passengers.

Transportation facilities are not limited to vehicles but also include provisions for pedestrians and public transport. This includes dedicated walkways, bicycle paths, bus bays, and rail stations. Each facility has its own capacity metrics; for example, subway systems may measure their capacity based on the number of passengers accommodated rather than vehicles.

Consider a subway system like a crowded stadium. The stadium has a set number of seats (capacity), just like subway cars have a limit on how many passengers they can hold. When a game ends, just like how passengers exit the subway, people flow out of the stadium. Proper planning ensures that both have the right facilities to manage large crowds efficiently.

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