Broken taxi meter scam

Taxi drivers—even legal ones with meters—will sometimes try to get a fat, unintended tip out of you

In many countries, the standard taxi fares and charges will be posted in any legitimate cab (like this one in Bologna, Italy).If the meter is not on, insist that it be turned on. Make sure it corresponds to whatever per-kilometer/per-minute rates are posted (most taxis post rates). 

In developed countries

In most developed countries, the initial "flag-fall" charge plus a per-kilometer amount (or charge for time stuck in traffic) is standard.

However, if you're taking a long trip—say, to the airport—taxis will often charge a flat fee, and this is legit. 

Before you leave, check what this flat rate should be at the airport's own website—or, failing that, with the local tourist office, hotel desk, or your guidebook.

In developing countries

In many developing countries, no matter where you're going, you almost always haggle a flat rate before getting in the cab.

Again, check your guidebook for the local norms.

Legitimate charges beyond the actual fare

The following small, extra charges (never more than $1 to $3) are usually legitimate:

• Charge per bag in the trunk.
• Charge for travel on a Sunday or national holiday.
• Surcharge for trips to/from the airport.
• Flat-fee charge if you called ahead for the cab (the flag drops when they get the call, not when you get in the car).
• Charge for trips after-hours (usually roughly after 10pm or midnight and before 5 or 7am).
• Tolls.

If none of those conditions applies, question any "extra" fee the taxi driver tries to foist off on you.