Catalan Traffic Department (SCT)
Evaluating the impact of variable speed limits on congestion, journey time and emissions.
The SCT had introduced a variable speed limit system on sections of the C31 and C32, two of the six access routes into central Barcelona.
The SCT contracted Aimsun to provide the modelling and evaluation of the already operational variable speed limit system on the C31 and C32 and its projected implementation on the A2 and B2.3
Aimsun used the Aimsun Next microsimulator to build two models of the study zone: one with fixed speed limits and an identical one with the algorithm for variable speed limits using an external Application Programming Interface (API) extension.
As the C31 and C32 already had an active variable speed limit system, we worked “in reverse” by calibrating the model with the system of variable speed limits and then deactivating the system and comparing the results. Aimsun Next shows both simulations side by side so in addition to comparative statistical results, there is a direct visual comparison of both models.
Aimsun analysed the traffic data provided by the SCT via licence plate recognition cameras, loop detectors and floating vehicles and filled in any information gaps with temporary pneumatic road tube detectors. For each of the four roads in the study Aimsun chose a number of weekdays from a spread of different times of the year with each detector taking a reading every 15 minutes from 6am-11am (the peak period of daily flow).
To evaluate fuel emissions, Aimsun used the Aimsun Next in-built emissions model, which computes CO2, NOX, PM10 and VOC emissions from instantaneous speed and acceleration. We defined the calibration parameters for each vehicle type and pollutant and computed the total emission by integrating this instantaneous emission into each vehicle journey.
The greater the congestion, the greater the benefits of variable speed limits. In situations with only mild congestion, variable speed limits can “over correct” and sometimes be counterproductive in terms of reducing journey times.
In all of the congestion hot spots across the network, variable speed limits made the journeys faster and smoother with fewer stops of shorter duration.
Fewer stops and less aggressive driving also led to a significant drop in fuel consumption and emissions. On the A2, which had a high concentration of HGVs, the reduction in emissions was as high as 20%.