(CNN) -- David Roberts remembers his days as a Denver police cadet in 1970: It was roll call, just like in the old TV dramas, and every cop was handed a "hot sheet" of vehicles totaling 10 pages, with license plate numbers printed in six columns.
It was a dizzying list, all single spaced. Every time Roberts saw a suspicious vehicle, he thumbed through the legal-size pages, seeing whether the car was stolen or wanted for use in a crime or impoundment for unpaid parking tickets.
"You can imagine how many times you would do that in a shift," said Roberts, who's now a senior program manager for the International Association of Chiefs of Police.
Each check, he said, amounted to "a fishing expedition."
That time-consuming process is now increasingly being replaced by technology called automated license plate readers. It's a system of high-speed cameras mounted on police or meter-maid vehicles that photograph license plates and run the numbers automatically against the 21st century version of a hot sheet, a database.
The nationwide emergence of this policing tool was highlighted this week in a report that portrayed the digital-era invention as shades of another Big Brother intrusion into American privacy, largely because there are no laws governing how long police can keep the data on vehicles, including those of innocent motorists, according to the American Civil Liberties Union.
But some law enforcement officials and advocates assert there's no need for state laws.
Groups such as the IACP encourage agencies to adopt policies to ensure the data is used for authorized purposes, with regular audits. There's been no major scandal of abuse since a study by Roberts found almost half of the largest police departments were using the technology in 2007.
The ACLU, however, calls the technology "a tool for mass routine location tracking and surveillance" of Americans and advocates legislation to ensure privacy.
"I understand the position they're taking on this," Roberts said of the ACLU. "I think agencies are very careful on how they use the data. Every agency I talk to and all the guidance we provide is to make sure that this is for official use only.
"If agencies have specific policies and articulate those policies and enforce them, it would obviate the need for legislation that would restrict this powerful investigative tool," Roberts said.
Moreover, other law officials argue, maybe license plate data from 10, 20 or 25 years ago could be used to track down a suspicious vehicle and solve a cold case.
"My first question is why," said Tyler Izen, president of the Los Angeles Police Protective League, the bargaining unit for 9,900 sworn officers ranked lieutenant and below. "And if the answer is because we have statute of limitations on crimes, well, there's no statute of limitation on murder.
"I would ask the ACLU if we could solve a family's homicide from 20 years ago, why would you want it destroyed?" he added, referring to the data. "Really what they're saying is they don't trust society to use that data reasonably, and I say we make sure we use it reasonably. I don't want to violate the Constitution."
Izen agrees that the data shouldn't be made available for purposes outside of law enforcement.
"I hear everyone's discussion about the right to privacy," Izen said. "If I'm telling my wife I'm somewhere, I certainly don't need somebody saying they saw my car somewhere else.
"I don't want a tabloid having access to the data telling me where every celebrity is located. That's not what we do," Izen continued.
That issue, however, did arise in Minnesota, according to the ACLU.
In 2012, the StarTribune newspaper in Minneapolis tracked the movement of the Mayor R.T. Rybak's car 41 times at license police readers in the prior year. The newspaper put the information on a map and gathered the data through public records requests.
As a result of that report, the mayor directed the police chief to recommend a new policy on data retention, the ACLU said.
The power of the technology seems commanding: A police cruiser can be mounted with cameras to check vehicles that are parked, in front of the patrol car or traveling in the opposite direction on the other side or the road.
Once the software confirms a match between a passing vehicle and the hot list, it emits a visual and audio alert on the patrol car's computer.
An officer, however, must double check the alert because sometimes the optical character reader software will misread a plate's "8" as a "B" or a "Z" as a "7," Roberts said.
A 2010 study by George Mason University found "there is little question that license plate readers are more efficient than previous (and, in many cases, current) police practices for checking license plates" such as the "look-out lists."
"License plate readers can continuously scan hundreds of plates in minutes without the officer paying attention to vehicles passing by or taking up radio airtime that might be used for more pressing communications. Because of these efficiencies, LPR may contribute not only to reduced discrimination in traffic stops, but also to reduced distractions and accidents while driving," the study said.
But the report raised questions on their effectiveness to deter crime -- especially when each license plate reader can cost $20,000 to $25,000.
"The most accurate license plate readers might be used by law enforcement officials in ways that have no specific or general deterrent, preventative, or detection effect whatsoever," the study said.
The question of deterrence was also raised by the Police Executive Research Forum and Mesa, Arizona, Police Department in a 2011 study that found the technology results in more arrests for car theft.
"We believe our results demonstrate that LPR technology holds a limited amount of promise for law enforcement. Some of the benefits include increasing the number of plates that the police can scan, increasing the number of 'hits' for vehicle theft and 'hits' for stolen plates, increasing the number of arrests for stolen cars, and increasing the number of recoveries involving occupied stolen vehicles," the forum study said.
"However, we did not find evidence that the LPR reduced actual vehicle theft rates for our targeted areas," the report concluded.
But deterrence isn't the principle objective of the technology, Roberts contends.
"Its principle objective is to identify vehicles that are wanted," Roberts added.
For the technology to become a deterrent might take time, Izen said.
"The short answer is, I don't think it does yet, but it may soon -- when we realize how much more easy it is to solve crimes," Izen said.