Even though I have never built a bomb (ignore all the people that keep asking me for help building one) I know a little about them. Tam gets the sarcasm right and now I'm going to fill in a few technical details for you clueless types:

• Hollywood does not do reality. Putting the time remaining until detonation in large numbers on a bomb is a Hollywood gimmick to increase tension in the story.
• Lights do not serve any purpose on a bomb other than to draw attention to it.
• If someone's intent is to hurt people or property drawing attention to the bomb is probably counter productive.
• Conventional explosives can only directly injury and kill via three different mechanism:
  • High speed projectiles, usually metal, that have been accelerated by the explosion
  • Overpressure which ruptures the lungs of the victims. You must be very close and sheltered from the high speed projectiles for this to matter
  • Acceleration of the victim. The various body parts are accelerated at different rates and the victim is torn apart or the victim can be thrown into an object that hasn't been accelerated; i.e. they are thrown against a concrete wall
• Bombs can cause indirect injuries such as the structural failure of a bridge, building, dam, dangerous chemical container, or starting fires (non-trivial but possible). Falling glass from the building above you is a big one to be concerned about.
• Surprisingly small amounts, fractions of a pound, of properly placed explosives can do amazing things to structures without the explosion hurting people just a few feet, even inches, away.
• Surprisingly large amounts (hundreds of pounds) of improperly placed explosives can do virtually nothing to structures and people who are relatively close by.
• Hollywood does not do reality. There are no safe ways to disarm bombs in general. Anything you can come up with I (or any other competent electrical engineer) can defeat such that either my bomb will detonate when I want it to or you make a bigger explosion than mine in order to destroy my bomb.
• Hollywood does not do reality. Fireballs are not an inherent part of explosives. It takes additional effort to create a fireball.  I've spent a lot of time figuring out how to make them (see also this page). It takes a lot of fuel to get something very interesting. The picture below used two pounds of explosives and four gallons of gasoline and I was clearly safe less than 50 feet away.

If you see something suspicious there are two things that are important; 1) How large is it? 2) What is it's placement?

Here are the evacuation distances based on the size of a bomb. Those are worst case distances based in part over the concern of broken glass from the windows between you and the bomb and on the buildings above the sidewalks. A few licorice string sized objects properly placed would be more effective in taking out a bridge than a car fully loaded with explosives driving across the top.

If the placement is very near some important structure such as a bridge or fuel tank one should be more suspicious than if it is in the middle of the Safeway parking lot.

Blinking lights on a flat panel attached to non-interesting structures are either not a bomb or evidence of a very stupid bomber. In either case it's not something to shut down a city's transportation about. Stupid bombers, with the exception of suicide bombers which aren't bombers but bomb delivery vehicles, are very rare because Darwin is very severe in his thinning of that herd. I just wish Darwin would thin the herd of stupid politicians as severely.