Time-space diagrams and signal optimization during the coronavirus pandemic: Improving traffic safety

  • Coronavirus (COVID-19) has generated unpredictable traffic patterns in many agencies. As stated in PTV Group blog post "Road safety during Coronavirus lockdowns" post, the combination of increased vehicle speed and a decrease in cyclist and pedestrian volumes introduce immediate safety concerns.  This trend diminishes the safety-in-numbers concept and drastically drops crash survivability for vulnerable users.  This requires prompt investigations to continue to meet Vision Zero goals of eliminating all road-crash deaths, not only reduce them. To thwart this issue, agencies needs to virtually test and deploy mitigations. In this example, I will discuss how you can use PTV Vistro’s time-space diagrams and traffic signal optimization to slow down drivers during the coronavirus crisis.

    PTV Vissim’s advanced microsimulation easily visualizes the effectiveness of your mitigations. Microsimulation empowers you to test further enforcement solutions like portable speed trailers. You may need approvals from your Agency’s management, city councils, and the public before deployment. High-end animations will boost the confidence of your stakeholders. Colorful KPIs and 3D animations will show how your mitigations like optimization of time-space diagrams will work on the ground. Now, let’s illustrate how your agency can use PTV’s versatile Urban Mobility System to increase traffic safety. 

  • Start by selecting your signalized corridor

    So, you’ve screened your jurisdiction’s traffic data. Likely, you’re finding that arterial speeds increased due to COVID-19 traffic patterns that were previously not achievable. These speeds are escalating due to lower traffic volumes, wide-open streets from less congestion, and time-space diagrams showing large green-tunnel bandwidths.  For this example, we will look at the following characteristics, which may represent your typical urban corridor traffic conditions:

    • An 85th percentile speed limit of 35 mph;
    • Excessive speeding is limited by congestion;
    • A 5-lane roadway cross-section;
    • Long pedestrian crosswalk lengths; and
    • 6-foot bicycle lanes.

    In PTV Vistro, we will use a 35-mph speed limit for our signal timing optimization. However, in PTV Vissim, you will want to set simulated “free-flow” speeds to something more excessive but feasible. We will set the desired speeds to 55 mph. This shows us potentially where motorists could speed to better target mitigations for pedestrian and cyclist safety. Of course, you can use your actual speeds from your coronavirus traffic data. 

Understanding your pre-coronavirus traffic conditions

  • PTV Vistro's pre-coronavirus analysis

    Before the coronavirus, analysis in PTV Vistro shows that the signal corridor needs an arterial green bandwidth of about 40 seconds. The time-space diagrams show that this advanced green time will clear long queues before the platoon's arrival. In the next image, we see that PTV Vistro shows 95th percentile traffic queues in the red and blue link hatching. Red indicates that queues could spill over into the previous intersection. Also, the Level-of-service (LOS) grades on the turn-movement arrows show the conditions at the intersection. LOS grades D, shown in orange and F, shown in dark red indicate heavy congestion.

  • PTV Vissim's pre-coronavirus simulation

    Next, we need a pre-coronavirus base model in PTV Vissim to compare coronavirus simulations. PTV Vistro converts to PTV Vissim. This quickly exports our model geometry and signal controllers. The traffic signal offsets in PTV Vistro’s time-space diagrams are transferred. In Vissim, we can insert a 55-mph free-flow speed on our corridor. Even with congestion, Vissim shows that there are some pockets where motorists could exceed the speed limit, especially between intersections 1 and 3 shown below. 


Coronavirus traffic conditions without mitigations

  • PTV Vistro's unmitigated coronavirus analysis

    So, what happens if there are no mitigations to curb coronavirus speeding? Under this scenario, traffic volumes are 65% of Pre-COVID-19 conditions. Using PTV Vistro we can see that the existing traffic signal patterns on the time-space diagrams support higher speeds. The next image compares the existing signal timings with a 35-mph arterial green band to a theoretical 55-mph band. Critically, the faster 55-mph green band fits on the time-space diagram. If there is no congestion or queues within the corridor, then these speeds are possible.

    To confirm the status of queueing and congestion, we can look at PTV Vistro’s operations. The 95th percentile queue lengths have decreased on the links shown by the blue blocks, and LOS grades are looking better. This suggests speeds may have increased.

  • PTV Vissim's unmitigated coronavirus simulation

    Going back to PTV Vissim, we can simulate the unmitigated Coronavirus conditions with existing time-space diagrams at free-flow speeds. We see that high-speeds are possible between intersections 1 to 3 and speeds have increased between intersections 3 and 4. These locations report a 5 to 10 mph surge in speeds. This is due to only the coronavirus traffic volume reduction. As we have learned in the previous article, higher speeds lead to a degradation in safety – requiring mitigation. One solution is to limit the opportunities to speed. Here we will utilize PTV Vistro’s time-space diagrams.

Mitigating coronavirus traffic conditions with time-space diagrams

  • PTV Vistro's mitigated Coronavirus analysis

    To mitigate the potential speed increases, we will utilize PTV Vistro. Vistro will reoptimize the corridor's traffic signal timings and green bands, delivering new time-space diagrams. This will right-size the corridor to the lower coronavirus traffic demand by shrinking the arterial green band. The goal will be to discourage speeding opportunities and bring vehicle speeds back to the pre-coronavirus levels.

    This mitigation begins by reducing the overall traffic-signal cycle lengths. With coronavirus traffic patterns, the lower throughput does not need as much green time. To ensure that the major street’s corridor is locked to a smaller green band, we need to set side-street signal groups to minimum recall. This removes the reside in green settings on the major street. Finally, we can optimize the signal corridor using PTV Vistro’s powerful route optimization tools.

    The next image shows the new 35-mph optimized green band in the time-space diagrams. This bandwidth is much smaller than the pre-COVID-19 conditions. Importantly, only a small 55-mph green band is present, but likely not achievable. Anyone wishing to speed has a higher likelihood to catch the next red traffic signal. With optimization and adjustments to the time-space diagrams, PTV Vistro shows that the corridor’s LOS grades and queue length are within acceptable ranges. These are improved operations compared to pre-coronavirus conditions. 

  • PTV Vissim's mitigated coronavirus simulation

    In PTV Vissim, we can simulate the green-band mitigation at free-flow speeds. We see that the new optimized time-space diagrams effectively slow down traffic compared to unmitigated conditions. However, there are still pockets, mostly between intersections 2 and 3 where speeding is possible. Let’s mitigate this with further enforcement tactics.

Mitigating coronavirus traffic conditions with speed trailers and time-space diagrams

  • PTV Vissim's mitigated coronavirus simulation

    We’ve learned about the importance of enforcement of speeds in PTV’s Bolivia study. Therefore, modeling speed trailers in PTV Vissim will simulate the impacts of driver feedback on current speeds. This driver response will help reduce speeds and harmonize traffic flows in the corridor. Using industry compliance data, we can assume that 65% of the motorists respond and slow to the speed limit after seeing speeds displayed on the trailer. Furthermore, an overall 80% of motorists slow down.

    In this scenario, we have applied the time-space diagrams – and then added speed trailers. Speed trailers are located to the left of intersection 1 and right of intersection 4. With speed trailers, we can revise the desired free-flow speed values at the trailer locations. These speeds will fit the curve of the trailers’ compliance rates.

    Results of this final mitigation using speed trailers and revised time-space diagrams show that the average simulated speeds return to the posted speed limit. Importantly, the likelihood of vulnerable users surviving a crash on this facility will be increased. 

Learn more about the Urban Mobility System

  • This experiment illustrates the timely issues that agencies are facing around the world from the coronavirus traffic patterns. Using the Urban Mobility System components, PTV Vistro and PTV Vissim, agencies can quickly test critical safety mitigations and make informed decisions concerning public safety.

  • Would you like to read about the benefits of the Urban Mobility System? Learn how to start with PTV Vistro!

    Click here to learn more

About the Author
  • Adam Lynch

    Adam is Senior Product Manager for PTV Vistro and a Certified Trainer at PTV Group. He has extensive industry experience as a Traffic Engineer/Planner using PTV Vistro for Traffic Impact Studies and Traffic Operations projects.  Connect with Adam on LinkedIn, or join the PTV Vistro LinkedIn Forum to learn more about PTV Vistro.