One of the most common indicators used in measurement and status report are traffic light indicators. Traffic light indicators are most commonly shown as a set of red, yellow and green lights. The metaphor draws from the nearly ubiquitous traffic light seen at nearly every intersection. Traffic light indicators are part family of indicators of that combine indices and scales. Indices are typically used when a single measure or metric does not tell the story. An index reflects a composite of measures. Measures and/or metrics are averaged together or combined using complex mathematics. The index is then transposed onto a scale so that it can be interpreted and used. For example, wind chill is an index that combines temperature and wind speed into a temperature perceived by the skin. Wind chill once calculated is shown on a temperature scale. As a project status indicator, a traffic light indicator typically reflects a synthesis of many attribute. The traffic light uses a simple scale in which red means trouble; yellow means caution and green means clear sailing. Traffic lights are adopted for three highly related reasons.
- Traffic lights are easy to recognize. The traffic light is a common symbol that every driver has been taught to recognize. Attaching a traffic light instantly indicates that a summary of status is being communicated.
- Traffic lights provide a consolidated view of complex attributes. The traffic light scale is a simple metaphor with three possible indications of overall performance. Even in a simple project attributes such as budget, client satisfaction and risk must be synthesized into single perception of status that can be communicated. Traffic light indicators force a synthesized view.
- Traffic lights are easy to explain. Once an organization reaches a consensus on the business rules that set a traffic light indictor to red, yellow or green, is easy to explain. Red is bad and requires immediate action, yellow means caution, performance issues require mitigation and green mean business as usual. Paul Byrnes, CMMI Lead Appraiser, when asked why people are drawn to traffic lights noted that “colors are easy…except for people that can’t see them… .”
Karl Jentzsch, a colleague at David Consulting Group summarized the case for traffic light indicators as “the appeal is that it provides an easily manageable number of ‘buckets’ to drop things into where the categorical distinctions are still fairly clear and inherently understood – good/go (green), bad/no (red), and in between (yellow).”
I often hear traffic lights defended with the statements like “we have always used traffic lights” and “or they are required by the PMO.” These are excuses that reflect an abrogation of thought and responsibility. It is too easy to succumb the simplicity of the indicator without reflecting on all the hard work and analysis needed to set the indicator. Typically this should be a lot of math and analysis to set the traffic light to red, yellow or green. The math and the analysis is where the real magic happens and requires thought and understanding. As an indicator, the traffic light is elegant in its simplicity; however that simplicity can also be its undoing.