SmarTek Systems SAS-1 Setup Using SAS Monitor - Manual (Page 17)

SmarTek Systems (www.smarteksys.com) 17
right area specifies the file name of an associated Header file (.hdr) that may be used when the
SAS-1 Archive Data File (.ahx) is converted to a specified end user format. Both of these file
names must be eight (8) characters or less and must be valid names relative to Windows file
name rules.
Most fields in the Archive Header Records correspond to standard required information for
traffic data collection and reporting, however, there are some parameters which affect the
performance and behavior of the SAS-1.
The Calibration Parameters for Vehicle Type, Prevailing Speed, and Average Peak value
Cap are used to calibrate how the SAS-1 counts three types of vehicles, how SAS-1 reports
speed during long periods with no vehicle detections, and how the SAS-1 limits the maximum
value of the Average Peak Power used for calculating the current detection threshold for each
lane.
For each observation period, the SAS-1 can provide vehicle count data for Long Trucks (tractor
trailers), Short Trucks (commercial trucks, trash trucks, dump trucks, etc.), and All Vehicles
(including trucks). The Calibration parameters for the Short and Long Trucks control how the
SAS-1 decides whether to include a detection in one of the two truck count categories.
Increasing the value of the Short or Long Truck Calibration parameter makes it harder for the
SAS-1 to count a specific detections as a truck. For example, if the SAS-1 appears to be over
counting Long Trucks in one or more monitored lanes, the Long Truck Calibration value for the
lanes in question should be increased. Properly setting the Long and Short Truck calibration
values for a specific site requires a reasonable amount of observation and comparison of the
count data against ground truth.
The SAS-1 reports and archives average vehicle speed for each lane. When there are no
detections during an observation period or over several observation periods (very rural areas or
late night times), there becomes a question of what to report for speed. Possible choices are to
report the speed as 0 mph (could trip incident detection algorithms), or report the last measured
value (could be several minutes old and does not reflect current conditions) , or report a specified
no data value (would likely not be compatible with most system software). SAS-1 processing
provides a means of tailoring the speed reported for the above no detection conditions using
three parameters (Prevailing Freeflow Speed by Lane, Speed Adjust Increment, and No Detect
TimeOut) stored in the Archive Header Records. If a time passes (No Detect TimeOut) without
a detection, the SAS-1 will begin adjusting the last valid speed estimate toward (up or down) the
specified FreeFlow Speed using the Speed Adjustment Increment. Setting the Speed
Adjustment Increment to 0 in effect disables the speed adjustment. Setting the No Detect
TimeOut to a large value in effect disables the speed adjustment or minimizes its affect.
For
SAS-1 firmware revision SAS_142 and higher, Archive Header Records must be set up
with reasonable values for these parameters and the header records sent to the SAS-1.
The SAS-1 keeps track of the Average Peak Value for detections on a per lane basis. This
ongoing calculation is used to compute the automatically adjustable detection threshold in each
lane. The actual limiting value for the Average Peak Value in each lane is the Average Peak
Value Cap parameter specified in the Archive Header Record plus the threshold value associated