Evaluation and Improvement of Roundabouts in Changchun

In Changchun, China, roundabouts are fairly common. They are often designed to connect the main roads because of their ability to reduce conflict points, making them safer than other intersections. They can also beautify the city when the central islands are landscaped. However, with increasing traffic, they may not function well and may even paralyze the road network. This means that it is important to evaluate the performance of roundabouts promptly, and to make necessary improvements if required. Using several roundabouts in Changchun as case studies, this article uses V/C ratio and delay to evaluate roundabout performance. Based on the result of evaluation, the micro-simulation model of the poorly-performing roundabout is built and enhancement is proposed.


Introduction
Numerous roundabouts are in use in Changchun, China, some of which designed to connect arterials.The roundabouts within the inner circle of Changchun is marked in Figure 1.In recent times, several roundabouts are at critical point of breakdown, experiencing prolonged delays and long queues.Without timely interventions to improve their performance, not only will the congestion problem deteriorates, also the failing roundabout may even paralyze the whole road network.Therefore, it is highly necessary to assess the current status of the roundabout performance and to propose suitable modifications to avoid breakdown.roundabout is built in order to analyze the detailed causes of poor performance and to explore alternatives to improve performance.A general process of evaluating a roundabout and assessing improvements is then presented.The framework of this paper is shown in Figure 2.
Since there are some differences among the different types of roundabouts chosen, an evaluation index based on V/C ratios and delays are used respectively for unsignalized roundabouts and signalized roundabouts.The corresponding level of service is then used to reflect the performance of each roundabout.Using data collected at the sites, the microsimulation model is then checked and calibrated.Proposed changes to the roundabout are applied to the simulated model to test if performance can be improved.

Classification of Roundabouts
Based on the existence of traffic light and the number of connected lanes, the common roundabouts in Changchun can be classified into four types: 1) Unsignalized four-leg roundabouts (e.g.Weixing Square and Jingyang Square) 2) Signalized four-leg roundabouts (e.g.Xi'an Square) 3) Unsignalized five-leg roundabouts (e.g.Zhengyang Square and Zhanqian Square) 4) Signalized five-leg roundabouts (e.g.Nanhu Square and Xinmin Square) An example of each type of roundabouts is used as case studies in this paper

Basic Data Collection
Table 1 shows the details related to the data collection plan.The evening peak periods are chosen as higher flows are present.

Unsignalized Four-Leg Roundabout-Weixing Square
The roundabout at Weixing Square is one of the more important ones in Changchun.
Figure 3 shows the configuration of the roundabout.
The evaluation index of unsignalized multi-lane roundabout is the V/C ratio of weaving section [1].This is computed using the geometric data from digital map and peak hour flow using the Index Calculator, as illustrated in Figure 4.
The corresponding levels of service based on the computed V/C ratios [2] are shown in Table 2.
As shown in Table 2, weaving sections 2 and 4 are at level of service F, indicating long queues and delays, implying that actions must be taken to improve the performance.
Using the Index Calculator, the V/C ratios and service levels are computed and shown in Table 3.
The roundabout is functioning acceptably though weaving sections 2 and 5 are near failure conditions.

Signalized Four-Leg Roundabout
Xi'an Square is one of the most important intersections in Changchun.As this is a signalized roundabout, the average delay is used to assess its performance [1].In this case, individual stop delay is measured.The level of service is determined based on the Highway Capacity Manual (HCM) [3].
The average delay and level of service at Xi'an Square are shown in Table 4.The roundabout has not reached capacity, although the traffic flow are rather high in Xi'an Square.

Signalized Five-Leg Roundabout-Xinmin Square
Xinmin Square is an example of a more complex signalized roundabouts in Changchun.The configuration of the roundabout is shown in Figure 6.
The average delay and service level of each leg is listed in Table 5.
The results show that during the afternoon peak, the traffic is high but the roundabout is operating just below capacity.

Basic Model Development
There has been past work done on simulating roundabouts [4] and Kotusevski, etc. [5] have compared several software in modeling roundabouts.Septarina [6] has made use of AIMSUN in modeling roundabouts in Sweden.In this paper, AIMSUN is used in developing the microsimulation model.
For the purpose of this paper, only the roundabout in Weixing Square is modeled to illustrate how changes to the roundabout can improve performance.Basic roundabout model has been established as is shown in Figure 7.
The traffic settings in the model are determined based on the traffic data collected.In order to define OD matrix and allocate the generation and attraction, centroid configuration is also necessary.Figure 8 demonstrates the four centroids of this roundabout.Blue lines represent generation, while green lines represent attraction.In this picture, every single line corresponds with a specific percentage.
The OD matrices for car and bus traffic are shown in Figure 9 and Figure 10 respectively.
A screenshot taken during the simulation process is shown in Figure 11.
The last step was to carry out error checking, which includes three stages: software error checking, input data error checking and animation error checking.
After checking the software and input data, animation is to be reviewed.Compared with the actual condition, the queue length in the simulation is slightly longer, which probably results from the default "offside priority rule".In addition, the traffic conditions in weaving section is slightly smoother than that in real-life scenario.This may be because the roundabout is affected by temporary construction within the central island resulting in a reduction in effective capacity in some weaving sections less than 5. Nonetheless, in general, the animation is very close to reality.

Model Parameters
AIMSUN utilizes the Gipps model for updating the vehicle movements.This is shown in Equation ( 1) below.( ) where: a n = max acceleration of vehicle n; b n = max deceleration of vehicle n; b n-1 = max deceleration of vehicle n-1; T = reaction time; θ = reaction time to ensure safety; v n = max expected speed of vehicle n in time t; v n (t) = speed of vehicle n in time t; x n (t) = driven distance of vehicle n in time t; S n-1 = effective length of vehicle n-1.Some of the parameters such as reaction time, speed limit, maximum acceleration, and maximum deceleration will need to be tested for sensitivity for the computation of performance criteria, e.g.queue length.From the earlier experimental runs, reaction time does not have a significant influence on average queue length.Moreover, given the "give way" protocol, the maximum give-way time should be taken into consideration.
The Root Mean Square Error method is used to obtain as the calibration criterion between the model measurements and the field measurements of V/C ratio.The calibrated parameters are shown in Table 6, along with the default values used in AIMSUN.

Simulation and Analysis of the Alternative Scenarios
Several scenarios to improve the roundabout performance can be experimented using the calibrated model.In this paper, two scenarios are discussed in the following.

Alternative Scenario 1: Addition of Lane in Weaving Sections
The roundabout can be improved by adding lanes in weaving sections.In this case, additional lanes are applied to the north and south weaving sections as shown in Figure 12.
The resulting changes in V/C ratios of each weaving section are shown in Table 7.
It is obvious that V/C ratios have dropped significantly in the north and south weaving sections, an improvement in service level from F to C.
Other performance measures have also improved more or less as is shown in Table 8.

Alternative Scenario 2: Apply Signal Control
In this scenario, besides adding lanes to the north and south weaving sections the

Figure 4 .
Figure 4. User interface of the index calculator.

Figure 11 .
Figure 11.Screenshot of the simulation effect.

Table 1 .
Data collection plan.

Table 2 .
V/C ratios and levels of service of each weaving sections in Weixing Square.

Table 3 .
The V/C ratio and level of service of each weaving sections in Zhengyang Square.

Table 4 .
The average delay and level of service of each entrance sections in Xi'an Square.

Table 5 .
The average delay and level of service of each entrance sections in Xinmin Square.