Traffic in London episode II: Predicting congestion with Tensorflow

Wednesday, November 02, 2016 Oliver Gindele

A few weeks ago Datatonic took part in a hackathon organised by Transport for London (TfL). At the hackathon we received data from road sensors in London which allowed us to build a nice real-time traffic visualisation which you can find in this blog post here.  You can also find more information about the hackathon in that post and some details on how we processed the raw sensor data with Dataflow (Apache Beam).

In this post we use the traffic data and build something quite different: A deep learning model that predicts congestion!

Visualising road traffic

The TfL data we use stems from sensors placed at traffic junctions all over London. These sensors detect if a car is above them or not every quarter second (see Episode I). As for the real-time analysis we use Dataflow to convert the raw bitstream from the sensors into two commonly used measures in traffic engineering: occupancy and flow. Occupancy simply tells us how often a sensor detects a car. It can be further divided by the average car length (around 4.5 m) to obtain the traffic density of a sensor. Flow on the other hand is a measure of the number of cars passing over a detector in a given time. Combining these two measures allows us to calculate the average speed per car (as flow/density), which we will use later to determine congestion.  If you want to learn more about traffic flow have a look here.

Before we attempt to model the congestion in London, we will first visualise some of the sensor data. This will give us an overview of the key measures and it will also help us to define and quantify congestion. Tableau is a great choice for this task since it offers convenient and great looking map views.




The figure above shows some of the traffic indicators plotted against each other. You can also track the time behaviour by looking at the colours. Overall, the figures agree nicely with the theoretical behaviour found here. We see for example that the speed decreases as the density increases (more cars on the road -> slower traffic).

Defining Congestion

To find out if a road is congested or not we need a single, robust quantity that describes the traffic state on that road. Finding such a measure is far from simple as we can illustrate by looking at flow: Flow is small for small values of density, but also for large values of density. This means zero flow can either mean a free road with no traffic or total congestions. To resolve this ambiguity we settled for speed as our congestions measure after some testing. In more detail we use speed/max(speed) as a congestion measure, since the absolute speed can vary largely between roads. Here, the max(speed) value is the maximum speed of a road, which will be usually achieved if there is very little traffic.

A relative speed value of 0 then means complete congestion and a value of 1 means the road is free. We plotted this congestion measure for small group of sensors in the map view of the above figure (top left). Such a congestion measure based on relative speed has been previously used by traffic engineers. However, it is worth noting that there is no ideal measure of congestion and many different definitions can be found in the literature.

Predicting traffic with the LSTM network

Now that we have a good measure for congestion we can try and predict its value using historic data. For this we build a deep learning model which uses the past 40 minutes of traffic data (relative speed) as input and which will then predict the congestion measure 40 minutes from now.

For the model we selected a LSTM recurrent neural network (RNN) which performs exceptionally well on time series data. We won't go into detail here about how the neural network works and how it is setup up, but you find a great article explaining LSTM networks here.  Our implementation of the model was done in TensorFlow which has built in functions for RNNs and the LSTM network. To simplify the problem we selected a small group of around 300 sensors out of the 12'000 sensors in London (the group can be seen in the map view above). 





We trained the model on a small set of only 8 days of traffic, but the results are already promising. The model can forecast the traffic 40 minutes into the future with good accuracy and it predicts the morning and afternoon rush hours nicely. The image below shows the predicted vs. actual speed for a few different sensors.







You can see how the neural network captures the different daily behaviour of each sensor individually. However, looking closely at the above graph you will find that some of the details of the time series data are not well reproduced. We also found that there are a few sensors that show a large overall prediction error. In order to increase the accuracy of the LSTM network it could be trained on much more data than just 8 days. Further, including more sensors might help training and of course all the sensors needed for a congestion prediction system for the whole of Greater London. Still, the model we have developed has a lot of potential and might help to do the following after some more tuning:

   +    predict future traffic state
   +    predict congested areas so they can be avoided or reduced
   +    detect incidents quickly

Final thoughts

The road sensor data we received from TfL is fascinating for two reasons: First, it is highly relevant as most us have to deal with some sort of road traffic during their everyday lives. Second, it allows us to analyse it in various different ways. In these two blog posts we leveraged the streaming capabilities of Dataflow (Apache Beam) to power a live visualisation and we forecasted congestion. Both are great use cases of the data and we hope that we are going to see more such datasets from TfL in the future.


Thanks to the great team at TfL for organising the demo and providing the data. 








2 comments:

  1. What is the benefit of this neural network approach compared to simply using congestion data from the previous day as a direct predictor of today's congestion ?

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  2. @Martin

    The traffic data is very seasonal on a daily, but also on a weekly level (maybe even over a year but we didn't have data for a whole year). Think about how the weekend traffic is very different from weekdays or how a Monday might be different than a Friday. If you just forecast based on some indictors from the last day you will miss out on these things. The neural network we choose (LSTM) is actually very good at including such long-term dependencies, so the model works quite well on a weekly (and possibly monthly) level.

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