Special event: 1st August Florence's downburst seen from a traffic sensor in the middle of storm

Around 7:20 PM of 1st August 2015 Florence has been hit by an unusual weather phenomenon: a strong downburst that caused a lot of damages in terms of eradicated trees, smashed cars and roofless houses. It has been a very violent phenomenon that changed forever the look of the southern part of the city.

Via Villamagna, beside being one of the most interesting site that hosts a traffic sensor of ours, is also at the center of the area that has been interested by the downburst. It is a one-way, two-lane street that lead from the suburbs to the boundary of city center in southern Florence. Here’s what the sensor recorded:

These plots show the traffic flow in Via Villamagna on a period of 24 hours centered at midnight. The plot above refers to the 1st and 2nd of August, while the plot below reports the same time interval from 18th and 19th of July, for comparison. The plots resolution (width of each bar) is about 10 minutes. It can be seen that there has been an abrupt drop of the traffic flow around 7:30 PM. Specifically, the two lowest bars between 6:00 PM and 9:00 PM in the plots cover the 20 minutes period from 7:16 PM to 7:36 PM.

If we look at how this traffic flow is distributed amongst the two most common classes, the result is even more interesting (from top to bottom, motorcycles and small cars):

Until 7:16 PM, the traffic flow is normal for both classes. Right after we observe a strange peak in class-1 flow and then almost no traffic for 20 minutes. In the following hours the flow of motorcycles begins to raise and reaches values that are even beyond what is usually observed in this time slot, while cars’ flow is not-significant. It is worth noticing that class 1 collects not only motorcycle passages, but also cycles and possibly pedestrians.

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