Back to Forum Page

APC Terminology
 

Term	Definition
Active IR	Near-IR light-beam sensors that transmit light continuously, or in pulses, and then receive that light. See Through-beam IR, Retro-reflective IR, and Diffuse-reflective IR.
Algorithm	Data processing technique for turning raw sensor data into passenger counts. See Finite-state machine, Neural net, and Image processing.
APC	Automatic Passenger Counting
AVL	Automatic Vehicle Location
AVLC	Automatic Vehicle Location and Control
AVL/CAD	Automatic Vehicle Location/Computer-Aided Dispatch
Contact APC	APC systems that such contact-type sensors, such as treadle switches, smart mats or switch matrices.
Crush Load	Passengers entering an already-full vehicle crowd into the door area, producing a crush load.
Data Retrieval	Retrieving the APC data from the vehicle after it is collected. See Offloading
Dead Reckoning	A navigation technique that uses speed and heading to determine location. Sometimes called, "ded reckoning," short for, "deduced reckoning."
Diffuse-reflective IR	Near-IR light-beam sensors which project a beam of light that reflects off an object, such as a passenger. See Active IR.
Event-based Data	APC data in which every passenger movement (on or off) is time-stamped or otherwise identified.
Finite-state Machine	Algorithm in which the raw sensor data causes the computer program to step through a series of pre-determined states to determine passenger counts.
GIS	Geographical Information System
GPS	Global Positioning System
Host System	On-board electronic system to which APC equipment is connected, such as AVL, AVLC, MDT, etc.
Image Processing	Algorithm which takes images from a vision system and processes them, using special electronic hardware or a microprocessor, into passenger counts.
Infrared (IR)	Can mean either near-IR - wavelength just slightly longer than visible light - or thermal IR - wavelengths between 5 and 10 µm.
Load	Number of passengers on a vehicle at a particular moment.
Machine Vision	See Vision system.
MDT	Mobile Data Terminal
Neural Net	Algorithm in which data filters through a probability matrix to determine passenger counts. Can be implemented using special electronic hardware, or programmed into a microprocessor.
Non-contact APC	APC systems that use non-contact sensors, such as active IR, passive IR, vision or ultrasonic.
Offloading	The process of retrieving data from a vehicle. This process may be manual, or automatic, using optical or RF modems, or a direct data link through the vehicle radio system.
On-board processing	Raw APC data is processed into passenger counts by a microprocessor on the vehicle.
Passive IR	Thermal-IR sensor that detects changes in temperature, such as moving passengers. Typically located overhead, these are similar to motion detector security devices.
Post-processing	Raw APC data is recorded on-board the vehicle and processed into passenger counts after data retrieval
Real-time APC	APC data is available at the moment it is collected.
Retro-reflective IR	Near-IR light-beam sensors with transmitters and receivers on the same side of the door area. Horizontal light beam is reflected off a retro-reflector on the opposite side of the door area. See Active IR.
RF	Radio Frequency
Smart Mats	Rubber mats with embedded optical fibers that sense deflection, and therefore, foot placement.
Stop-level Data	APC data which reports the on and off passenger counts at each stop, and may include the load at the end of the stop.
Stops Database	Database of stops that is related to geographical and route/schedule information.
Switch Matrices	Rubber mats with a array of many embedded switch elements
Tare	Passenger load at a particular reference point, such as the end of a trip. Used for establishing a reference load for real-time APC systems.
Time-based Data	Data in which passenger counts are accumulated during discrete time intervals. No stop or location information is recorded.
Through-beam IR	Near-IR light-beam sensors with transmitters and receivers on opposite sides of the door area. See Active IR.
Treadle Switches	Rubber mats containing embedded switch elements.
Ultrasonic Sensor	Acoustic sensor which senses passengers by transmitting and receiving ultrasonic pulses.
Ultrasound	Sound frequencies above the range of human hearing, typically 30 to 100 kHz.
Vision System	System in which an electronic camera takes pictures of the door area to determine passenger counts.

Why APC?

Need for APC is determined by requirements for scheduling & planning, reporting, and fleet operation. Here are some typical uses:

· Total counts for fare verification

· Collecting ridership data for scheduling and planning

· Schedule adherence 

· Real-time load for operational control

Data Collection & Retrieval

The APC data has to go somewhere. For real-time APC, it is passed to a host system where it is transmitted to a central location over the vehicle communications link. In many APC systems, the "store and forward" type, data is stored on board the vehicle for later data retrieval. It can be stored in digital memory, in a removable data card, or on a diskette.

Data retrieval can be manual or automatic. Manual data retrieval, sometimes called "sneaker net", requires that someone visit the bus and remove something (a data card or diskette), or plug something into the APC equipment, like a notebook computer. By equipping the vehicle with an RF or optical modem, the APC system can automatically connect with an external computer and download the APC data without human intervention.

Location Information

To make stop-level data meaningful, the data has to have location information. Otherwise, how would you know which stop is which? Unless the APC equipment is connected to a host system with location capability, APC needs another component. This other component could be an RF or optical signpost receiver, a GPS receiver, or perhaps dead reckoning equipment.

Signpost systems use strategically placed RF or optical transmitters to send a code to the passing vehicles. The codes are stored along with the other APC data to determine the vehicle location through post-processing. Of course, this requires that the vehicle have an on-board RF or optical signpost receiver.

Alternately, each vehicle is equipped with a GPS receiver. Position is determined by receiving signals from 3 or more satellites. The receiver performs the complex calculations, and the APC system stores with information along with the APC data. This can be more complicated than it sounds, but doesn't need the signpost infrastructure.

Dead reckoning can be used by itself, or with signpost or GPS equipment, to provide location information. The vehicle's odometer signal, plus signals from an electronic compass or other heading indicator (like a gyroscope) are processed to determine the vehicle's velocity (speed and direction). This information is stored along with the APC data to determine vehicle location through post-processing.

Accuracy

This is complex subject, because it involves statistics. APC accuracy can range from around 80% to better than 95%, depending on the type of APC equipment, the configuration of the vehicle door area, the types of loads, and the behavior of passengers. Accuracy is always better when loads are lower, and worse when loads are high.

Generally, accuracy is measured by comparing APC data to manually-collected data. Manual data, which is hard to collect in heavy load situations, may only be 90 to 95% accurate, making it difficult to evaluate accurate APC systems. Video taping, followed by careful analysis in the comfort of the office, is the most reliable method of evaluating accuracy.

Sometimes, the accuracy specified for an APC system is stated as absolute - "The APC system must be 95% accurate." This is not a meaningful statement, because it does not specify a confidence level for this measurement - a common trap in statistics. Put simply, any system will encounter rare conditions that will cause it to make mistakes. An APC system may be really accurate, but every once in a while, the accuracy will be terrible, and the data should be thrown out. If it desired that the system be 95% accurate, but it is acceptable to throw out 5% of the data (usually on a trip basis), a better way to specify the accuracy is, "the APC system must be 95% accurate, 95% of the time".

Specifying an APC System

Here are some things to think about before specifying an APC system.

1. What is the purpose of the APC system?
As obvious as this sounds, for the APC system to meet the customer's needs, the customer must know the problem to be solved. Since APC is automatic, lots of data will be available, but to what end?

2. Will the APC equipment be connected to a host system?
If so, interface and data format requirements must be carefully thought out.

3. Is stop-level data required?
In many cases, stop-level data is not required and the APC equipment can be greatly simplified. Stop-level data also implies the need for location information, creation of certain databases for post-processing, and, in general, greater complexity.

4. What accuracy do you really need?
Many APC systems are over-specified, resulting in higher cost and complexity than is really necessary. For example, real-time systems, such as AVLC, that need APC load information only, can generally get by with lower accuracy, providing the system has a way to tare the load at strategic points.

5. How many APC-equipped vehicles are necessary?
For AVLC systems, where real-time load is desired, each bus needs to be equipped with APC. However, where the APC data is to be used to scheduling and planning purposes, a rule of thumb is to equip 10% of the fleet.

6. How will the data be retrieved?
Manual data retrieval may be fine for smaller fleets, but consider automatic data retrieval for larger systems.

7. How will the data be processed?
APC equipment is automatic! That means data is being collected whenever the APC-equipped vehicles are in service. This can result in massive "data glut." Consider the amount of data to be processed, the equipment required to process it, and the programming effort required to create or support the system. Some APC implementations have failed because the data processing requirements were greatly underestimated.

8. How many people will be needed to retrieve, process the data and generate reports?
While it's true that, with an APC system, you won't be sending many people out to collect data in the field, the number of people required to manage the system, retrieve and process the data, and generate reports may be significant, depending on their skills and the data processing software.

9. Consider all the costs
APC systems need not be excessively costly, compared to other similar investments in equipment and personnel. In fact, a properly specified and operated APC system will produce a net cost savings through lower personnel costs and increased operating efficiency. However, consider all the costs involved, including acquisition, installation and life-cycle costs of the equipment, plus system and data management.
 

Conclusion

Those transit authorities that have installed and used APC systems generally feel that they can't get along without them. However, like any new method, specifying, installing and learning to use an APC system takes money, time and effort. We hope this little tutorial has at least helped to illuminate some of the darker corners of the APC world.

Back to Top