As this report from the Department of Transportation makes clear, congestion on our roadways causes travellers to add "buffer time" to their planned journeys. So, for instance, one may have to allocate 32 minutes for a trip that would have taken 20 minutes in uncongested traffic if one would like to guarantee on-time arrival. The 12 minutes would either become time spent sitting on the road or wasted time due to arriving too early.
Buffer time can be applied to graphs too. Some graphs require readers to spend time fishing out the information. The chart used to illustrate travel time belongs to this category. The clock analogy fails; in fact, it confuses matters as the hour hand just sits there serving no purpose. The buffer time between staring and comprehending is too much!
Only four numbers underly this chart: travel time when uncongested and buffer time to guarantee on-time arrival, for 1982 and 2001. The following version gets to the point without fuss. It shows that the travel time increased significantly even under uncongested traffic; worse, the buffer time multiplied.
Reducing buffer time is always good but some buffer time may be inevitable. In the traffic analogy, to eliminate all buffer time would mean lots of unused capacity. In the context of graphs, more complicated charts would require more time; the key is whether the reader is rewarded for the time spent figuring out the chart.
Source: "Traffic Congestion and Reliability", Department of Transportation.