> Home > Photo Enforcement Program > Frequently Asked Questions

Frequently Asked Questions

How, and how often, is a fixed speed detection system calibrated and checked for accuracy?

Detection system accuracy is established at the time of installation by comparing the speed indicated by the system with a calibrated detection device. The system is configured to an accuracy of +/- 1 MPH of the calibrated detection device reading. The term "calibration" is often used inappropriately in this situation. There are no adjustments made once the initial setup is accomplished, and the only way to change the way the system detects vehicle speed is by changing one of the two constants involved (#1 the distance between the two sensors, and/or #2 the way time is measured by the computer).

A variety of devices are used to verify accuracy and include but are not limited to:

  • Vehicle speed test and verification
  • Radar detection systems
  • Certified speedometers
  • Laser speed detection systems

The city's vendor conducts verification of system accuracy routinely for each speed certified location to ensure proper equipment operation. The routine inspections referenced are also conducted when maintenance repairs are performed. In order to put a repaired system back in operation a systems check is performed, and if the indications of improper speed exist, a speed verification check is performed as part of that inspection. These indications include but are not limited to:

  • Visible damage to the sensors
  • Excessive detection "abortions" due to failed speed tolerance checks

The system performs a speed tolerance test for each detection and ensures the speeds entering and exiting the detection zone are consistent or the detection aborts the processing of the detection as a violation.

Speed verification checks are performed as a result of the following activities:

  • Initial installation and commissioning
  • Sensor replacement
  • Sensor cabling or connector replacement
  • Detection system replacement
  • During routine inspections for proper system operation

return to top

How is speed determined at fixed detection system locations?

Two piezo strips are used to time a vehicle's location at two specific fixed points. Based on the time it takes a vehicle to travel from point A to point B, a speed calculation is made. A loop is placed between the two piezos to verify activation by a vehicle (inductive loop detects the presence of metal) rather than another source.


  1. There are two piezo strips "cut" into the road at a fixed distance apart (approximately 10-15 ft)
  2. Timing is started when the vehicle strikes the first piezo sensor which is verified by the rise of the loop
  3. Timing stops when the same "axle" strikes the second piezo
  4. Each "axle" is counted and measured for speed until the fall of the loop which signifies the end of the vehicle
  5. Each independent speed measurement (all subsequent axles relative to the first) are compared to the first axle speed
  6. The detection is valid if all speeds are accurate within preset tolerance limits

Once the initial conditions have been met (steps 1 -3) the camera is activated, when all conditions have been met the system verification is completed and the detection packaging sequence commences firing the cameras based on the vehicle's speed.


return to top

How does the City of Scottsdale determine speed limits?

The City of Scottsdale follows the guidelines set forth in the Manual on Uniform Traffic Control Devices (MUTCD) and the Arizona State Statutes to set speed limits in our city. The State Statutes allow local authorities to determine speed limits on streets under their jurisdiction based on an "engineering and traffic investigation." The MUTCD provides the criteria that is used for the engineering study. The MUTCD states that the speed limit should be within 5 mph of the 85th percentile speed of free flowing traffic (the speed at which 85% of the vehicles are traveling at or less than). It also allows you to consider road characteristics, roadside development, parking and pedestrian activity, and crash experience. We do most of our studies based on citizen requests, in areas where the roadside environment is changing due to development, or where we have experienced a higher number of accidents than usual.


return to top

Do photo enforcement flash units cause collisions?

No. The flash duration is less than 1/2000th of a second. That is shorter than lightning due to weather. The intensity of the flash is no brighter than any camera one might purchase for personal use. The height of the pole, shape of the reflector, and distance to motor vehicles and pedestrians also minimize the effect of the flash.

In Scottsdale alone, over 500,000 photo enforcement incidents have been photographed using flash since 1997. In all that time there has not been a single reported incident of flash causing a collision. If anything, photo enforcement flash has a positive effect on driver behavior. It provides instant feedback to those who speed or run red lights, and most react by slowing down.


return to top

Why are speeders allowed an 10 mph buffer?

The margin is intended to address language in the Arizona Revised Statutes related to speed offenses that defines, "...reasonable and prudent..." speed. It also provides a reasonable margin of error/grace for inaccurate vehicle speedometers and short-term lapses of attention by otherwise law-abiding and generally cautious drivers.

Our primary focus is on intentional, aggressive and negligent drivers. Statistics tell us that at 10 mph over the posted speed limit, the probability of being involved in a crash doubles. However, at 20 mph over, the probability increases to 11 times greater. We are trying to change the driving behavior of those persons who are most likely to cause crashes.


return to top


Are license plate sprays and covers that are advertised and sold to prevent photography of plates by photo detection cameras prohibited by law?

Arizona law (ARS 28-2354) requires that, " A person shall maintain each license plate so it is clearly legible." The sprays and covers themselves are not illegal. The act of illegible display of the plate is.


return to top

Are points assessed against my driver's license for photo enforcement convictions?

Yes. In accordance with Arizona Administrative Code R17-4-404, Driver Point System, points are assessed by MVD for each conviction reported to them by the Court. Each red light conviction is assigned two points, and each speed conviction is assigned three points.


return to top

Can I avoid points being assessed against my driver's license for photo enforcement convictions by attending a defensive driving class?

Yes. If you have not used the Defensive Driving Class (DDC) option in the most recent two years, you can avoid points being assessed against your license. This alternative is fully explained on the options page of each photo enforcement citation.

Additional Resources