Summary and Conclusions
7.1 Summary
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lateral movements. Gradual lateral shifting has been introduced, unlike the instantaneous lane change observed in homogeneous traffic.
Updating procedures and the corresponding parameters depend on the cell structure of the CA model. Since the cell structure was changed in this study, its effect on the updating procedures and the corresponding parameters was thoroughly analyzed.
The parameters of the present CA model have been arrived at based on the previous researches, field observations, and parametric analysis. Based on the parametric analysis, some of the parameters have been modified. The impact of variable lateral gap and interaction headways were found to be prominent. The developed model has been macroscopically validated using the field observed speed, flow, and occupancy data of two different road sections. The macroscopic relations of the observed and simulated data were matching fairly. Simulated free flow conditions are further validated by comparing the observed and simulated free flow speed distributions. Model’s ability to simulate the congested traffic conditions has also been validated by comparing the observed and simulated queue formation and dissipation characteristics. The queue formation and dissipation speeds obtained from the simulation model were comparable to the field observed values.
The developed model was used to analyze the effect of road width (specifically two lane roads) being used in urban regions. It has been observed that slight changes in the road width, between 6.9 to 8.4 m, significantly influence the traffic stream characteristics.
7.2 Conclusions
7.2.1 Trajectory correction
Empirical observations are crucial in understanding and modeling the traffic behavior.
Analysis of the vehicular trajectories leads to a fair understanding of the traffic behavior and for this to be successful trajectory data need to be fairly accurate.
Trajectories obtained from TRAZER were found to be erroneous and need to be corrected before proceeding to data analyses.
1. In this study, a methodology based on complete ensemble empirical mode decomposition with adaptive noise (CEEMDAN) is proposed to smooth the trajectory data.
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2. From the error analysis it has been found that the CEEMDAN approach was relatively better compared to the existing approaches in smoothing the trajectory.
3. It was found that successive smoothing is essential to estimate the accurate speed and acceleration from the smoothed trajectories.
4. Wavelet Transform(WT) using ‘gaussian’ wavelate function (for CWT) and
‘spline’ wavelet function(for DWT) were found to be more accurate in estimating the instantaneous speed and acceleration from the smoothed trajectories.
5. Internal consistency analysis of the position and speed also proved the suitability of proposed method (WT) for speed estimation.
7.2.2 Lateral gap model
Lateral gap maintaining behavior of vehicles moving in no-lane-disciplined heterogeneous traffic depends on sevaral factors. Total lateral gaps (gap on both the sides of a passing/overtaking vehicle) maintained by four types of vehicles, namely, LMV, MTW, MThW, and HMV have been modeled in this study.
1. Field observations suggest significant variability in the total lateral gaps even when the passing/overtaking vehicle is traveling at a constant speed.
2. A vehicle maintains a minimum lateral gap with its adjacent vehicle at zero speed and a maximum lateral gap beyond certain speed limit. Keeping in view the minimum and maximum lateral gaps, Logistic curve model was found to be more suitable in representing the vehicular total lateral gap maintaining behavior.
3. It has been observed that the speed of the subject vehicle, size, and the speeds of both the adjacent vehicles affect the total lateral gaps.
4. It was also found that the size and speed of the adjacent vehicles influence the total lateral gap only when the subject vehicle is traveling beyond certain critical speed. This speed is more than 30 km/hr in case of LMV and more than 20 km/hr in case of the other vehicles.
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5. In case of relatively wide roads, the impact of side vehicles was found to be prominent even at lower speeds of the subject vehicle.
6. Linear trends have been observed between various threshold speeds and the road width.
7.2.3 Traffic characteristics
1. It has been observed that in no-lane-disciplined heterogeneous traffic the following vehicle is not only influenced by the LV, but, also by the other vehicles such as, LFLV and RFLV.
2. Effective LV is the closest longitudinal vehicle among FLV, LFLV and RFLV.
3. Lateral shifting of a vehicle depends on the availability of lateral gap considering the impact of side vehicles in addition to the other safety criteria based on available back vehicle.
7.2.4 Simulation model
Cell width of the present CA model is modified based on the minimum lateral gap requirement of vehicles and the physical width of smaller sized vehicles such as MTW and MThW. Cell length of the CA model was decided based on the mechanical characteristics of vehicles, such as accelerations etc. Lateral gaps were represented explicitly, i.e., the lateral gap required by a vehicle is not included in the physical width of the vehicles.
1. It was found that even the LFLV or RFLV, if longitudinally closer to the SV compared to the LV, sometimes hamper the movement of SV. In this case one of these two vehicles would be the effective leading vehicle. In this scenario the SV may reduce the speed based on the available road width ahead and pass the LFLV or RFLV rather than following the LFLV or RFLV.
2. The interaction headway was found to be significantly influencing the traffic stream behavior. Increasing interaction headway reduces the maximum flow.
It indicates that, in no-lane-disciplined heterogeneous traffic, the effect of the
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leading vehicle is not felt on the following vehicle unless both the vehicles are longitudinally closer. This may be due to the reason that the freedom available for the following vehicle to adjust its lateral position. It can be in the form of completely shifting to the other available part of the road or veering away slightly.
3. Results obtained based on the various lateral gap models indicate that the gap maintaining behavior has significant impact on the macroscopic relations.
Constant (but low) lateral gap model results in overestimated capacity flows.
4. The final simulation model has been validated for two different road sections.
The simulated macroscopic relationships were found to be statistically matching with the observed macroscopic relationships.
5. Two-dimensional Kolmogorov-Smironov (2D-KS) test between observed and simulated distributions indicates that the simulated distributions of flow- occupancy and speed-occupancy are statistically similar to that of the observed joint distributions.
6. Model results have also been validated at microscopic level in terms of comparisons between the observed and simulated FFS distributions corresponding to various vehicle types.
7. Ability of the model to replicate the congested conditions has been verified in terms of queue formation and dissipation. Queue formation and dissipation speeds obtained from the model were similar to that of the field observed values.
7.2.5 Effect of road width
The simulation model developed in this study has been utilized in studying the effect of two-lane road widths, presently being used in urban regions in India, on the traffic flow behavior.
1. It was observed that changing the road width from 6.9 m to 7.2 or 7.5 m has marginal effect on the maximum flows crossing the road section, in case of both heterogeneous traffic and cars only traffic.
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2. The change in the maximum flow of heterogeneous traffic stream was significant in case of road width increasing from 7.5 to 7.8 m.
3. Increasing road width from 6.9 to 7.2 m or 7.5 to 7.8 m has significant effect on the congested flows of heterogeneous traffic stream. The difference in the congested flows was marginal in between 7.2 to 7.5 m and 7.8 to 8.1 m.