- Presentation
- Local public transport
- Five proposals
- Final proposal
- Working group
A roundabout and an indirect maneuver to drain traffic
Characterized by the stressful urban problems of vehicular congestion, the route of via della Bufalotta underwent (by Roma Servizi per la Mobilità on behalf of the Department for Mobility Policies, which has strongly supported the project, and on behalf of 7th Mobility and Transport Department of Rome Municipality) a study of traffic in order to transform the critical issues of the studied area into transport-related solutions of efficiency able to render the intersection safe and improve the conditions of vehicular flow.
Current framework
The route of via della Bufalotta, at the intersection with the Gronchi viaduct, is marked by the conjunction of a particular level of vehicular flow with roads regulated by an intersection with a stoplight. The streets all have different characteristics: while via della Bufalotta is a single-road, single-lane route that allows movement in both directions, via Fucini is wider and is divided into two lanes, one for each direction, separated by a tree-lined median strip that currently is used improperly for parking. Also the Gronchi viaduct features two-land roadways separated by a divider and, unlike via Fucini, the road has characteristics more similar to an extra-urban expressway. Finally, at the intersection examined, there are pedestrian crossings on both sides of via della Bufalotta and via Fucini but these are missing from the side of the Gronchi viaduct.
The intersection appears therefore characterized by different roads, with characteristics peculiar to each, which are traveled every day by major traffic flows.
Critical areas
The axis of via della Bufalotta, over 3 km long in the stretch inside the GRA (Great Ring Road), has various different sections (or has lights whose timing can be controlled and changed by a centralized traffic control office), with several signalized intersections and roundabouts. In particular, at the signalized intersection where the Gronchi viaduct and via Fucini intersect with via della Bufalotta, three main critical areas have been detected, which are detailed below:
- while the Gronchi viaduct and via Fucini both have separated roadways with each direction endowed with three lanes, each of them dedicated to a specific maneuver such as a left turn, straight and right turn, via della Bufalotta, with a single carriageway, have a single lane for both allowed maneuvers of driving straight on and turning right.
- the intersection, moreover, is regulated by a traffic light system in three phases with reduced spaces for the accumulation and conducting of some of the allowed maneuvers; the smallness of this area causes congestion on the approaches of via Fucini, along the Gronchi viaduct and along via della Bufalotta;
- the intersection of via della Bufalotta and the Gronchi viaduct is regulated so as to allow all maneuvers except left turns by vehicles coming from via della Bufalotta in both directions, coming from the historic city center and coming from the GRA. This prevents blockages in and facilitates a greater vehicular flow. However, along that stretch infractions continue to occur by vehicles that recklessly make left turns, generating criticalities at the intersection.
To all this we must add the fact that the area under consideration, within the quadrant between the Southern stretch of via della Bufalotta and the Gronchi Viaduct, has elements of strong attraction consisting mainly of many shops and schools.
It must therefore be considered that those items have inevitable repercussions on circulation, particularly at the peak hours of the morning and afternoon, creating particularly evident phenomena of vehicular congestion.
The general framework of analysis performed on the Bufalotta - Gronchi - Fucini area thus highlights the substantial difficulty of outflow of vehicles, which causes phenomena of congestion during peak hours of the day and that therefore has need of immediate planning interventions.
Basically, all the interventions are developed in the following manner:
- starting from celerimetric surveys of the state of the locations,
- we then passed to manual vehicle counts carried out on weekdays, and at times in the morning, afternoon and evening;
- in conjunction with the survey of the development of medium and maximum vehicles queued in line at traffic lights;
- we also carried out the survey of the traffic-light cycle (121 seconds during the day and 105 seconds at night);
- there were detected the various bus lines affecting the hub considered;
- finally, we proceeded to the study of the design solution.
The traffic data from the survey
At what times is traffic more intense? The Agency carried out a campaign of monitoring traffic on the route of via della Bufalotta, which also covered the intersection in question. The manual surveys of traffic flows, made in working days in the month of December 2010, involved three time slots in which all passing vehicles were detected and classified. The maneuvers monitored are depicted in Figure 2.
The phenomena that generate queuing, and vehicular congestion, are instead presented in the subsequent figures, starting from Fig. 3, which highlights the transport demand bearing on the different approaches of the intersection at rush hour in the morning. While monitoring the flow conditions at the intersection, states of congestion were detected on all approaches during peak hours. Especially in the morning the situation appears critical for the flow of vehicles exiting the Gronchi viaduct and for vehicles on via della Bufalotta heading towards the center.
In the afternoon there was a worsening of the situation for vehicles transiting along via della Bufalotta towards the GRA and for vehicles exiting via Fucini, while the situation improves for vehicles on via della Bufalotta towards the city center and for vehicles exiting the Gronchi viaduct, whose flow is always quite congested.
The numerical values, expressed in equivalent vehicles (equivalent vehicles: a representative measure of traffic that equates the cars, through appropriate homogenization coefficients, to their respective vehicle categories), represent transit flows through the intersection and the vehicles that mean to pass through the intersection but that, not succeeding, produce queuing phenomena (lines) on the different approaches.
The analysis carried out has thus highlighted the strongly critical areas of the intersection, favoring the preparation of a project aimed at reducing waiting times for vehicles. Roma Servizi per la Mobilità has carried out a feasibility study that explored six possible transport-related solutions, described in detail in the following paragraphs.
However, among the many projects presented, only the last one, for reasons which will be described later, proved more effective than the current solution, and technically feasible. A decision was made therefore to draw up the preliminary draft for this project.
However, before examining it, it is necessary to consider the phenomenon of vehicular congestion which manifests itself, in the current state, around the intersection.
Traffic queues
Undoubtedly one of the most relevant criticalities at the intersection studied concerns the phenomenon of traffic queuing that affects via della Bufalotta the most, in both directions, but is also present on the Gronchi viaduct and on via Fucini. The following offers a graphical representation of the traffic queues that form at the intersection in its current state:
The traffic queues detected in the morning at the intersection amount to approximately:
- via della Bufalotta (towards the GRA), 140 m of queues
- via della Bufalotta (towards the center), 260 m of queues
- Gronchi viaduct, 140 m of queues
- via Fucini, 100 m of queues
PUBLIC TRANSPORTATION
In this regard, it is important to get a picture of what the lines of local public transport are that travel through the present scenario to understand how they will change once a design solution is presented, and how the future scenario will evolve, particularly in view of the changes that are going to affect it.
There are five local public transport lines which make use of the intersection in question:
- 86 - travels along via della Bufalotta
- 308P - travel via della Bufalotta
- 335 - travels along via della Bufalotta
- 342 - travels along via Fucini and the arm of via della Bufalotta heading towards the city center
- 341 - travels along via Fucini and the Gronchi viaduct
There are four line stops near the crossing, affected by the intervention: two on via Fucini and two on via della Bufalotta towards the city center. Routes, lines and bus stops are shown in the diagram below.
FIVE PROPOSALS
THE FIRST PROPOSAL: longer green lights for those travelling through the intersection
From an initial survey of the intersection in question a hypothesis was formulated that mainly insisted on optimizing the existing traffic light installation, without making changes to the timing scheme of the lights. Thus, on the basis of the quantities of vehicular traffic taken into consideration, the green light times would be optimized, while the yellow and red times (4+2secs) would be normalized, making the cycles and light times of the nearby traffic lights homogenous, and also verifying the minimum times for pedestrian crossings.
This first solution therefore sought to encourage the flow of vehicles by shortening the queuing of vehicles near the intersection, the representative effect of which, as we know, is mostly recognized in traffic congestion.
In particular, with the implementation of the new traffic light, one could partly streamline the queues on the only lane of via della Bufalotta but at the expense of the Gronchi viaduct. The latter, endowed with several lanes and roadways separated by a median, could thus have absorbed the higher vehicular load that was partially being taken away from via della Bufalotta, which would not in any case have benefited in a conclusive way from the intervention.
While seeking to optimize the traffic flows at the intersection, this first solution would not really have made significant reductions in the average wait times or in the formation of queues at the intersection, in the end proving ineffective in real terms.
SECOND PROPOSAL: a traffic light with differently distributed phases
The project involved the reorganization of the traffic light's timing system. For this purpose, the plan also provided, for vehicles coming from via Fucini that must turn left toward via della Bufalotta, the realization of a dedicated lane in abutment, through the widening of the roadway and the removal of a high-trunk tree.
Current
stoplight
phases
The reorganization of the timing of the traffic light system provides for the modification of two of the three current phases. While, in fact, the first would coincide with the current one, a reorganization of the second and third phase would have allowed straight/right turn maneuvers from via Fucini and the Gronchi viaduct (phase 2) and left turn maneuvers for vehicles coming from via Fucini and the Gronchi Viaduct, and driving straight along via della Bufalotta (phase 3). By doing so, one could assign a longer green light time for straight and right turning maneuvers for vehicles coming from via Fucini and the Gronchi viaduct, recovering this time from the green light time dedicated to left-turn maneuvers carried out by a smaller number of vehicles.
The plan's
stoplight
phases
In addition to these measures, the project foresaw the construction of an opening for a U-turn on the Gronchi Viaduct that would allow an indirect left-turn maneuver from via della Bufalotta towards via Fucini. The objective of constructing the opening dedicated to a U-turn maneuver on the Gronchi viaduct was to diminish the backup of vehicles on Via della Bufalotta, which were blocking the intersection. The second proposal, too, although bringing about clear benefits at the intersection, did not appear to offer a definitive solution to the problem since it reduced but did not eliminate the problem of vehicular congestion.
With the second proposal, by maintaining the 3 phases but combining the left-turn maneuvers from the Gronchi Fucini route into a single movement, the queueing times could have been shortened on the concerned routes, as expressed in the following figure.
With the modification and optimization of the traffic light timing arrangement at the intersection in question, there would have been a reduction in the average delays and in the queueing of vehicles. In particular, on:
via della Bufalotta (towards the GRA)
an 18% reduction of queues;
via della Bufalotta (towards the center),
an 4% reduction of queueing;
Gronchi viaduct
an 8% reduction of queues;
via Fucini
an 20% reduction of the queues.
There is no doubt that this intervention would have allowed a reduction of the average queues as compared to the current scenario, thus facilitating the flow of vehicles passing through the area.
However, the solution provides for a modification to the software of the timing diagrams inside the traffic light headquarters that must be set to the new timings. In addition, the plan requires an estimate for the costs relating to the preparation of some civil works capable of achieving a waiting lane for making left turns from via Fucini in conjunction with the opening of a U-turn on the Gronchi viaduct.
The second hypothesis, then, would improve the intersection but would create a financial burden due to the works to be made and still not guarantee a definitive solution to the problem.
THE THIRD PROPOSAL: one less turning maneuver
In the third proposal, it was decided to change the timing of the Fucini - Bufalotta stoplight, reducing the phases to 2, by deleting from the traffic light installation the left turn maneuvers on the Gronchi Fucini route and their deferment onto U-turn openings, within a range of 100-150m.
By reducing the traffic light phases and streamlining the U-turns through the elimination of the left turn from via Fucini, we have seen that, while a decrease is achieved in average delays and queueing on the other three routes, queueing is found on the same via Fucini, broken down as follows:
via della Bufalotta (towards the GRA)
an 50% reduction of queues;
via della Bufalotta (towards the center),
an 8% reduction of queueing;
Gronchi viaduct
a 43% reduction of queues;
via Fucini
twice as much queueing.
Conversely, however, problems as the elimination of congestion are detected on via Fucini, because of the magnitude of flows of vehicles making U-turns, the limited width of the central box and of the longitudinal traffic flows present that somehow slow the normal flow of traffic.
From a preliminary test, it was also found that, during the intervention phase, the queues on via Fucini could increase even to as much as 150m upstream of the U-turns, with a 100% worsening of the circulation compared to the current state.
THE FOURTH PROPOSAL: a roundabout instead of a traffic light intersection
Following the third proposal, the opportunity was evaluated to act with several infrastructure projects that would provide alternatives to changing the traffic light phases. In particular, the idea was devised to realize a roundabout and it was decided to test immediately the maneuvers of the intersection by inserting a ring able to guarantee the execution of all the current maneuvers, including those currently prohibited at the intersection. Then in order to ensure the necessary exchange roads, the proposal envisaged the deployment of road input routes in support of the current situation while the traffic flows used would remain the same as those used at present.
The point in favor of the implementation of the fourth proposal would undoubtedly be the implementation of a roundabout instead of traffic lights, through which would reduce the waiting times that the same causes normally. There would then be an "automatic" management of the fluctuations of vehicular traffic, no adjustments required. The phenomena of queuing would be further diminished, also resulting in decreases in average delays. However, functionally, this hypothetical solution did not prove to resolve the situation, as all the vehicles entering during peak hours, would not be able to be managed completely with a classic roundabout: here too, albeit to a lesser extent, queueing would occur at the approaches to the roundabout. Furthermore, the approaches to Via della Bufalotta, though requiring greater functionality, turn out to be the most disadvantaged ones since they are situated along the secondary route.
THE FIFTH PROPOSAL
Hypothesis A: a "mini roundabout" with an underpass on via della Bufalotta
The fifth proposal evaluated the efficacy, in terms of fluidity, that would result from the elimination of the vehicular component crossing via della Bufalotta, through the introduction of a vehicle underpass that, starting from the stretch immediately upstream of via Guerrini, would reach all the way to the Liceo Scientifico Nomentanol.
For turning maneuvers and for those crossing Fucini - Gronchi, there was thus designed a roundabout with a lower impact on the territory and on land use. By doing so, we could have taken advantage of the natural contours of the land, with ramp slopes of 5.5%.
The elimination of the traffic light system would allow the final arrangement of the intersection, at the entrance to which in the ring we would be able to effect significant reductions in the traffic lights, previously encountered with other solutions.
The elimination of the traffic light system would allow the final arrangement of the intersection, at the entrance to which in the ring we would be able to effect significant reductions in the traffic lights, previously encountered with other solutions.
Among the negative effects, however, attention should be paid to the interventions that would impact Art.11 on the closure of the access road to Olivetti, with the construction of a new lane on the Gronchi Viaduct, and to the question regarding the proximity of the buildings to the road and the underpass that would be built along the Bufalotta route. It should also be pointed out that, even in this solution, pedestrians would be crossing in the presence of traffic. To overcome these drawbacks, it was thought, therefore, to revisit the idea of a mini-roundabout, with some changes.
Hypothesis B: a "mini roundabout" with an underpass on Fucini - Gronchi
In the alternative solution of Proposal B, the plan provided for the re-sizing of an island roundabout able to guarantee the execution of all the exchange maneuvers (turning, entering, exiting the roundabout) that one might foresee for the crossing, excluding only the flows of traffic crossing the Via fucini - Gronchi viaduct route, for which the plan provides for the construction of a specific underpass.
Among the strengths which would contribute to improving the intersection, there doubtless should be counted the elimination of the traffic light and especially the opportunity to exploit the space occupied by the median of Via Fucini and the Gronchi viaduct to build an underpass at a suitable distance from the dwellings.
However, several drawbacks to this plan were detected, many of which were already present in the previous solutions, including: impacts on Art.1, the closing of the access road to Olivetti, with the construction of a new lane on the Gronchi Viaduct, and finally the possible direct arrival of traffic flows from the Gronchi viaduct which would cause difficulties in the Fucini-Niccodemi hub. One must also take into consideration the possibility of queuing of vehicles in the underpass, with a significant increase in exhaust gas emissions and their impacts on the environment.
The fifth proposal, although not technically feasible, had identified the functional solution which allowed for the eliminattion of the traffic light intersection and at the same time foresaw the construction of a medium-sized roundabout able to distribute the remaining traffic adequately. From the functional point of view, it was therefore decided to design a solution that would unhinge the maneuver of crossing Via della Bufalotta (towards the Center) and that at the same time would manage not to further occupy the roundabout. From this consideration springs the solution that has been deepened in the preliminary design.
FINAL PROPOSAL
A roundabout with a turning maneuver
The design solution that was finally identified essentially consists in the construction of an interchange at the intersection of the Gronchi viaduct - via Fucini - via della Bufalotta, consisting of a roundabout and a further "U-turn" on the Gronchi viaduct connected to the roundabout by two exchange sections.
The geometric characteristics of the roundabout provide for an outside radius of 24.5 meters, a two-lane ring (4 meters wide each lane) for a total of 8 meters, and an inner surmountable quay 3 meters wide. The plan also provides for four maneuvering lanes outside the roundabout, dedicated to right-hand turns. In reality, the roundabout provides only three entrances, thus ensuring that one cannot enter the ring from the direction of via della Bufalotta towards the center; these vehicles will have to follow the path along the Gronchi viaduct to then take via della Bufalotta. In addition, the exit from via della Bufalotta towards the center is reserved for public transport during rush hours in the morning and evening, while at other times it can be used also by private traffic.
The functional element that guarantees the continuity of via della Bufalotta (towards the city centre) consists of two roadways serving as exchange lanes: the first, further north, with 4 lanes, serves to allow the vehicles coming from the roundabout to trade places with those coming from via della Bufalotta; the second, further south and consisting of three lanes, serves to allow vehicles coming from the viaduct to change places with those coming in the "return" direction, connected through the two exchange lanes.
Permitted routes
from via della Bufalotta North (GRA) to via della Bufalotta South (center)
from Bufalotta North to via Fucini
Route 1
Vehicles coming from via della Bufalotta (towards the Center) that are headed into the next exchange lane of via della Bufalotta, after entering the viaduct, travel along the first exchange lane, and the return route, and enter the second exchange lane.
Route 2
Vehicles coming from via della Bufalotta and headed into via Fucini, after entering the Gronchi viaduct and having travelled along the first exchange lane, arrive at the "return" route, and enter the second exchange lane. To get onto via Fucini they must enter the ring and use the second exit of the roundabout.
Route 3
Vehicles coming from via Fucini and that are directed towards via della Bufalotta GRA toward the GRA do not need to enter the ring but can enter via della Bufalotta simply by using the lane dedicated to right-hand turns.
Route 4
Vehicles coming from via Fucini and that are directed towards Via della Bufalotta towards the Center, must enter the roundabout, continuing towards the Gronchi viaduct, then enter the first exchange lane and turn around using the "return" stretch and then take the second exchange lane. In the hours in which the inner lane is reserved for public transport, they must exit the roundabout using the outer lanes, to enter into Via della Bufalotta. In the remaining hours they will enter into the roundabout directly and then take the third exit.
3. from via Fucini to via della Bufalotta North (GRA)
4. from Via Fucini to Via della Bufalotta South (center)
5. from via della Bufalotta South (center) to Via della Bufalotta North (GRA)
Route 5
Vehicles coming from via della Bufalotta South have at their disposal an entry lane in the roundabout that allows them, thanks to the direct exity, to resume via della Bufalotta immediately towards the GRA.
6. from the Gronchi viaduct to via della Bufalotta North (GRA)
7. from the Gronchi viaduct to via Fucini
8. from the Gronchi viaduct to via della Bufalotta South (center)
Route 6
Vehicles coming from the Gronchi viaduct must enter the roundabout and exit on via della Bufalotta towards the GRA.
Route 7
Vehicles coming from the Gronchi viaduct must enter the exchange lane, turn onto the roundabout and turn and then head straight on via Fucini.
Route 8
Vehicles coming from the Gronchi viaduct must simply stay on the outside lane and, without entering the ring, take the side lane that enters via della Bufalotta, towards the Center.
The Pedestrian Routes
There are specific pedestrian routes that complete the project. The realization of the latter is to occur on via Fucini and via della Bufalotta in the north and south directions.
The crossings will be barrier-free thanks both to the presence of adequate ramps connecting the sidewalk and the roadway, and due to the "cut" of the raised traffic islands that conform the specific lanes to the vehicular traffic.
It was not considered necessary to provide a pedestrian crossing for the Gronchi viaduct, but a new pedestrian crossing has been designed in via Fucini, essentially placed at the service of the TPL stops, as a result of their relocation to a position farther from the intersection, in anticipation of the implementation of the project of the roundabout.
The system of pedestrian crossings on via della Bufalotta, south of the roundabout, also permits easy access to the stop placed in the reserved lane from the network of pedestrian footpaths; considering, then, the expected length for said stop, it was deemed appropriate to provide a second crossing placed at the southern end of the quay.
None of the pedestrian crossings have traffic or pedestrian crossing lights and, except for the crossing that services the LTP stops on via Fucini, each is placed on roadways with a lane width of 4.50 meters.
Public Transportation
Lanes used for surface public transport, through the implementation of the new project, will see the start of serveral interventions. In particular, the intervention provides for the displacement of the stops on via Fucini in a position such as to allow the vehicles to pull up alongside on another, in safety, as they come from the viaduct (line 341) and to allow a smooth maneuver of entrance to/exiting the roundabout for vehicles travelling on the via Fucini (line 342).
The reorganization of the bus stops on via della Bufalotta in the direction of the GRA has to be reorganizated through the creation of a new sidewalk and a semi-gulf at the stop.
The bus stop on via della Bufalotta towards the center is instead scheduled to be located on the exit lane from the roundabout and organized so that it can serve two local public transport vehicles simultaneously, in order to avoid public transportation from hindering the flow of the roundabout.
The routes of the lines that currently make us of the intersection are not modified, merely undergoing variations in the lengths of their routes due to the geometric-functional reorganization of the intersection. Of particular note:
lines 341 and 342: no changes
lines 86, 335 and 308P: the route towards the GRA remains unchanged;
The route towards the center has an increased length of about 420 meters, due to the presence of the U-turn on the Gronchi Viaduct.
The benefits
The realization of the final project would bring about several benefits, especially on the circulation and travel times within the area concerned. Here below, we offer details of the most significant benefits:
average speed at rush hour (traveling through the intersection): 25 km/hr after the realization project, versus the 12 km/hr of the current scenario;
total travel time (intersection): currently, 1,350 hours daily, 730 with the new project, with a reduction of more than 50%;
distances: 18,000 km per day versus the current 16,000 km (a 13% increase), caused by the lengthening of the route towards the center for vehicles coming from Via Della Bufalotta (direction GRA);
cost of the operation (excluding compulsory purchases by the distributor): 1.3 mln euro.
The simulations of the final project
The figures show two still frames extracted from a simulation model created specifically to test, step by step, the effectiveness of the design solution presented. In the figure above, the focus is on the roadway viability diagram of vehicles using, both entering and exiting, the new turning maneuver designed for the Gronchi Viaduct. Instead, below we have wished to focus your attention on the circulatory diagram and on the maneuvers of entry into and exiting from the roundabout. Attached to this document, the video of the simulation is available, that lets you evaluate the project as a whole.
The Working Group
Mr. Carlo Riccucci
Mr. Giacomo Tuffanelli
Lorenzo D'Amore
Text drafting
Mrs. Valentina Zelada
In Charge for Operational Unit of Transportation Systems Planning
Mr. Stefano Brinchi
In Charge for Mobility Projects, Planning, Innovation
Mr. Alessandro Fuschiotto