The three primary products of the aerial traffic surveys are:
In the fall of 1998, Skycomp conducted a series of aerial photo-surveys of highway traffic quality in the metropolitan Atlanta planning region. The purpose was to obtain traffic quality information to support regional planning activities. Using the mobility and vantage point of fixed-wing aircraft, a photographic inventory and quantitative measurement of traffic quality was made on the backbone of the transportation system. Approximately 500 miles of highways were selected for the survey, and included all of the freeways approaching downtown Atlanta, plus several major signalized highways (View Survey Area Map). These selected highways were photographed during morning and evening peak periods of commuter travel (View a detailed description of the Survey Methodology on the Methodology tab). The result was a set of photos and data representing average traffic conditions of the surveyed highways as of autumn 1998. The result was a set of photos and data representing average traffic conditions of the surveyed highways as of autumn 1998. This data formed a base line for future comparisons so that the effects of changes on the system could be monitored.
In the fall of 2001, the survey was repeated on the same set of highways using the same methodology. Again, the result was a set of photos and data representing average traffic conditions on the surveyed highways. Also, with a second set of data points, it was now possible to make comparisons related to the 1998 baseline survey.
As the value of this monitoring program became clear, a decision was made to extend the primary network of highways out to the boundaries of the larger (21-county) Atlanta planning region. Extending the boundaries of the survey region involved approximately 250 miles of freeways that had not yet been surveyed. In the spring of 2002, these extended segments were photographed, utilizing the same methodology of the previous surveys.
In the spring of 2004, the scope of coverage was extended further to include an additional 1,500 miles of high-volume signalized arterial highways from throughout the 21-county planning area. This regional arterial network, together with the extended primary network, forms the backbone of the region's state highway transportation system.
In the fall of 2005, the extended highway segments from 2002 were added to the original segments from 1998 and 2001, and the combined network (approximately 750 miles) was surveyed once again.
Survey coverage of the entire system (approximately 2250 miles) was repeated in the fall of 2007 and the spring of 2008. During the spring and fall of 2010 the entire system was surveyed once again, this time covering 2650 miles of highways in the Atlanta metropolitan area (approximately 400 miles of highway was added to the system in 2010).
In the fall of 2002, the aerial photo traffic survey program in Georgia was expanded to include the cities of Augusta, Columbus, Macon and Savannah. Approximately 950 centerline miles of highways were selected, to include all freeways and major arterial highways. Designated highways were photographed during morning and evening peak periods of commuter travel. The result was a set of photographs and data representing average traffic conditions as of fall 2002. These data formed a base line for future comparisons so that the effects of changes on the system could be monitored.
In the fall of 2010, the survey was repeated using the same methodology. The combined centerline mileage of surveyed highways was expanded to 1100 miles (View survey area maps for Augusta, Columbus, Macon and Savannah). Again, the result was a set of photographs and data representing average traffic conditions on the surveyed highways. Also, with a second set of data points, it was now possible to make comparisons related to the 2002 baseline survey.
Aerial photography was selected as the best means of collecting mobility data across the large planning regions.
During survey operations, 100% overlapping aerial photographs are acquired of all surveyed highway links at a frequency of one sample per hour, repeated weekdays between 6:30 and 9:30 a.m., and between 4:00 and 7:00 p.m. So that the effects of incidents can be identified and removed, and to reduce the effects of daily fluctuations, coverage is repeated over four different weekday mornings and evenings. After flights have been completed, photography is segregated by highway, and counts are made of the number of vehicles operating in each segment, in each direction. Raw data are entered into a database file for computation of densities, levels-of-service, and surrogate levels-of-service (the latter for interrupted-flow highways).
Once performance measures have been computed for all flights, data scrubs are conducted to screen for anomalies by comparing the results, segment by segment, across different days and time slices. This process is used to identify the effects of incidents and other non-recurring events that affect traffic quality. Where confirmed or suspected incidents / events are found, corresponding data are tagged for exclusion from the averaging process. Final (averaged) performance ratings, therefore, reflect normal traffic conditions, without the effects of non-recurring events. It is these ratings that form the technical basis for the analyses in the survey deliverables.
For uninterrupted-flow highways such as freeways and toll roads, the density of vehicles on each link is first determined by taking counts from the aerial photographs; the units of density are passenger cars per lane per mile, or pcplpm. (Trucks and buses are converted to passenger-car equivalents for this calculation.) Once average density values have been determined for both directions of each link, they are converted to level-of-service (or LOS) ratings based on the following conversion table:
|LOS||Traffic Flow Description|
|0 to 11||A||Very light traffic|
|12 to 18||B||Light traffic|
|19 to 26||C||Moderate traffic|
|27 to 35||D||Moderate to heavy traffic without significant slowing|
|36 to 45||E||Heavy traffic with minor slowing|
|> 45||F||Congested traffic involving slowing and stopping|
By reformatting this table as a scale and adding color, it becomes more clear how the segment densities and Levels of Service translate into the colors used in the maps (the red and orange arrows) and reports.
Density values and level-of-service ratings form the basis for analysis of uninterrupted-flow highways. Graphics are also prepared which use colors to depict various levels of density values, ranging from very light traffic (green) to severely congested (red). This table serves to translate the colors used in the report graphics.
Use of the LOS scale makes it easy for non-technical persons to appreciate the nature of the flow on each highway link. For this reason, LOS is promoted in the 2000 Highway Capacity Manual (HCM 2000) as a recommended way to convey the nature of traffic flow to stakeholders and decision-makers.
HCM 2000 also defines a way to determine LOS for the other primary type of facility, interrupted-flow highways (characterized by signalized intersections). However, for signalized highways density is not a suitable measure because interruptions caused by traffic signals serve to cluster vehicles and release them in groups (platoons); this causes density measurements vary wide when conditions are fundamentally the same. Instead, HCM 2000 uses average travel time as the primary basis for defining LOS on this type of highway. This type of data is often collected by driving instrumented cars through the traffic stream, called the ‘floating car technique’. However, average travel time cannot be cost-effectively measured from photographs taken from fast-moving aircraft. Instead, a surrogate methodology for approximating arterial LOS has been developed by Skycomp, Inc. This surrogate methodology uses the presence and population of vehicle platoons, and the degree of queuing at signalized intersections, to define the six surrogate LOS ratings:
|Traffic Flow Description|
|A||Very light traffic flow|
|B||Light flow without clearly defined platoons|
|C||Platoons less than 15 vehicles per lane (vpl)|
|D||Platoons between 15 and 25 vpl|
|E||Platoons > 25 vpl, or queues > 20 vpl at no more than 2 signals|
|F||Three or more signal queues > 20 vpl; or one severely congested signal > 40 vpl|
NOTE: These values are underlined to emphasize that they are surrogate LOS measures, not true HCM 2000 LOS measures.
As with the freeway level of service table, these surrogate levels of service can also be displayed on a scale with color corresponding to the colors used on the maps (the red and orange arrows) and reports.