Normally if you see police officers and yellow tape around a house or a scene, something bad has happened.

One of the handiest tools in the trunk of a police cruiser is the roll of police line tape. The bright yellow plastic tape may say POLICE LINE DO NOT CROSS or may say CAUTION.

Both warnings seem ridiculously easy to understand. Why would anyone cross through the tape and into crime scenes or dangerous areas?

Many officers ask this question to themselves or their partner right before they grab the unfortunate who has wandered into a restricted area.

Did you know that you can be arrested for interfering, hindering and opposing a police officer for crossing a police line? That charge carries a maximum possible sentence of 2 years in prison.

In the police academy, officers are taught that once they arrive on scene, they “own” that scene. The officer will be held accountable and responsible for anything good or bad that happens at that scene. If they decide to string yellow police line tape up, rest assured there is a good reason.

Police line tape may be used to keep citizens away from a hazard. Downed power lines, gas leaks, fire scenes, barricaded gunmen or any number of hazards may require cordoning off an area.

People are astonished when “their” street and sidewalk are blocked. Citizens will often demand entry into the restricted area because they are homeowners near the hazard or they do not want to travel out of their way.

Some people get angry about it. Those people have to be verbally directed not to enter the scene. Some have to be physically barred from the area, which is the final step before arrest.

Officers do not want to arrest a citizen at these police lines, but the officer guarding that police line is responsible. If the officer allows a cranky citizen into a restricted area and that citizen gets electrocuted by a downed power line, blown up lighting a cigarette near a gas leak, or shot by a barricaded gunman, the officer will be held accountable. That officer who “failed to protect” the citizen could be civilly liable.

The most common use of the tape is to preserve a crime scene. It is forensically important that only the people who are collecting evidence be allowed in a crime scene, so the scene is not inadvertently contaminated.

"CSI" aficionados understand that when people enter a crime scene, they leave evidence behind and take evidence from the scene with them. Technically speaking, this is called Locard’s Theory of Transfer. That is why traffic through a crime scene must be restricted to the investigators actively working the scene.

It seems incredible, but some people try or actually cross police lines. One of the most difficult crime scene lines ever staffed by the Ann Arbor Police Department was at the Blockbuster Video on Jackson. There had been a murder inside and there were police cars, officers, crime scene tape and television news trucks all over, but people were in a frenzy to avoid late fees on their video rentals. They parked in the bank lot nearby and simply walked up to the tape, lifted it, and strolled right toward the front door.

These customers appeared proud that they were ingenious enough to avoid the pesky police cars blocking the driveway and were returning their rentals on time and therefore avoiding late fees.

They were not as proud of themselves when officers nearest their entry point ran up asking for identification and demanding to know why they were entering a crime scene. The stern officer then informed the miscreants that since they had entered a homicide scene, they may have to testify in trial why they invaded a clearly marked crime scene.

It got so bad at that scene, which we held for several days because of the complicated forensics within, that patrol officers staffing the police line kept a large box for customers to deposit their rentals. It should also be noted that owing to the tragedy, Blockbuster waived late fees.

When approaching an officer staffing a police line, you may certainly ask what is going on. However the officer may or may not give you any explanation. If an officer has time and there is little danger, he or she will give you a general explanation.

If however the officer is hiding behind a large oak tree with firearm in hand, do not be offended by a rather sharp and perhaps colorfully vulgar answer — the officer is merely trying to get your immediate attention and emphasize the danger.

In that case, it's best to run — not walk — in the direction the officer has pointed.

Lock it up, don’t leave it unattended, be aware and watch out for your neighbors.

Rich Kinsey is a retired Ann Arbor police detective sergeant who now blogs about crime and safety for AnnArbor.com.