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WMATA Real-Time Parking
Information Feasibility Study
Chapter 5: OPTIONS AND PREFERRED
SYSTEM FOR WMATA
Space-level Detection
Conceptually, space level detection is very similar to zone level detection – similar
detection technologies monitor vehicles. However, there are some very important
differences between space level and zone level detection. Primarily, zone level
detection counts moving vehicles, while space level detection monitors when each
space is occupied. Detection of moving vehicles is generally a more mature and proven
technology, and is widely used in traffic monitoring. Occupancy detection requires the
continuous monitoring of a fixed space for the presence or absence of a vehicle. Once a
space is occupied, it is expected to be occupied for many hours.
This distinction implies a simpler task for the detectors. Rather than reading a loop
detector 60 times a second, as is typical for applications that measure traffic flow, a
space detector can be read every ten seconds or so. However, most detectors are
designed to detect changes in a state, and it can be difficult for them to note a change
and record that the “base” condition is now different. Sometime the detectors may be
simplified units compared to typical on-street detectors, since they need only detect
whether a vehicle is occupying a small, well-defined area.
With zone level detection, a handful of detectors are used to monitor hundreds or
thousands of spaces. With space level detection, hundreds or thousands of detectors
are often required. At an accuracy level of 99.9%, a detector may miss three cars a day
driving past the sensor entering a large facility. The space level detectors have the
advantage that they will have multiple opportunities to verify occupancy, so accuracy
should be even higher. However, the detectors have to be maintained. If there are 4000
detectors within a facility, there are 4000 points of failure, 4000 sensors that have to be
monitored and potentially provided with recurring maintenance (preventative as well as
required). While most detectors are designed to last for many years, it is expected that a
few of them will require maintenance (in the case of video, all of them) or replacement
every year.
For space level detection, the system no longer needs to directly compare moving
vehicles against a database – the system simply tallies how many detectors do or do not
detect a vehicle. This could be matched to a map of the facility that shows graphically
each of the detectors and whether they are occupied. This option provides an easy way
to determine when detectors are failing; an operator simply has to drive or walk through
a facility and see which space is showing the wrong occupancy.
Space level detection also creates a communications issue as potentially hundreds or
thousands of detectors must communicate with the controller. This may require a
significant amount of additional cable and conduit, or increased wireless spectrum.
Page 5-14 June 2009