An Integrated Design Framework for Safety Interventions on Existing Urban Roads—Development and Case Study Application
Abstract
:1. Introduction
1.1. Background on Quantitative Methods for Road Safety Interventions
- NSPF = average predicted crash frequency from the SPF for a specific road element class, such as in the urban case: segments, three/four-legged, signalized/unsignalized intersections, roundabouts (possibly referred to specific severity classes) (crashes/year);
- L = length of the road segment (km), which is not present in SPFs for road intersections;
- AADT = annual average daily traffic volume, possibly disaggregated in multiplicand components for considering diverse traffic units (motor vehicles, cyclists, and pedestrians) or the importance of the segments intersecting at road intersections (major/minor roads), and having separate coefficient estimates for each volume component (motor vehicles, cyclists, or pedestrians/year);
- Xi = set of predictor variables related to road, traffic features, environmental variables, and context variables;
- βi = set of regression coefficients, which include β0, β1, and β2 as well.
- (crashes/year), average predicted crash frequency, eventually adjusted for local conditions through the application of CMFs on the road element;
- = average observed crash frequency on the same road element (crashes/year);
- statistical weight assigned to the , dependent on the over-dispersion parameter k of the associated SPF and the total predicted crashes in the period (-).
1.2. Background on Qualitative Methods for Road Safety Interventions
- Safety audits (design stage), for new road projects or the enhancement of existing roads, by checking that road safety criteria are respected in the road project;
- Safety inspections (management stage), for identifying safety issues on road elements, by highlighting sites needing interventions based on qualitative scores and judgments.
1.3. Objectives
- Can quantitative and qualitative methods be integrated in a framework that is flexible enough to be applied in different contexts of safety interventions on existing roads?
- What are the possible problems arising from the application of such an integrated framework, by also taking into account the different sources of data needed?
- Which solutions may be provided for the problems that emerged during the application, useful for the future application of the proposed method to other cases?
2. Methods
2.1. Methods: End of Network Screening Stage
2.2. Methods: Diagnosis Stage
2.2.1. Reconstruction of road geometry
2.2.2. Individuation of homogeneous road elements
- Depending on the reference predictive method, which will be used for the safety predictions (Equation (1)), a sub-segment can be considered as homogeneous if all the road, traffic, and context variables included in the predictive method are reasonably not varying within it. If this step is undertaken at the early design stage, this may simplify the safety prediction applications.
- Traditional variables related to the horizontal and vertical alignment (e.g., radius of curvature, longitudinal/cross slopes) could be overlooked for defining homogeneous urban sub-segments. The rationale for this is that such variables are typically not considered in urban safety predictions (see e.g., [42,43]), possibly depending on the lower speeds and the less demanding urban topography. Clearly, there could be some urban cases in which this assumption may fail, and those variables should be taken into account instead.
2.2.3. Reconstruction of crashes
- It contributes to numerical EB estimates of the road site crash frequency (Equation (2));
- It summarizes the site-specific safety problems that have resulted in crash outcomes.
2.2.4. Identification of the road function and related criticalities
2.2.5. Check of road geometric standards
- Checks of sight distance at intersections and driveways, especially in residential contexts with several possible visual obstacles;
- Checks of sight distance with specific regard to collisions with VRUs (see e.g., [46]).
2.2.6. Road inspections
2.2.7. Reconstruction of boundary conditions
2.3. Methods: Selection of Countermeasures
2.3.1. Selection of safety measures
- Critical analysis of each safety issue from the diagnosis stage.
- Selection of possible alternative safety measures supported by studies that may document their positive effect on safety. These sources include the CMFs database (e.g., [5,11,17]), systematic reviews based on meta-analyses [4] that also include cost–benefit indications (see also [49]), and examples of solutions that take drivers’ factors into account [50].
- Critical comparison of findings related to the same safety measures, by taking into account the specific environment of each study (urban/rural, two-lane/multi-lane roads), the crash types and severities considered, and the robustness of the methods employed (i.e., through reliability ratings, see [17]). In fact, the CMFs (or functions) that have been developed for some of the specific conditions listed above may be not applicable to different contexts, and they may be actually very different (e.g., CMFs either greater or smaller than one for the same measure).
- Assessment of possible transferability issues dependent on the degree of infrastructure development, and on geographic and socio-economic factors.
- Assessment of the capability for each single measure (deemed as potentially applicable) to solve different identified safety issues in a consistent way. Measures that are able to solve safety issues at a project level should be preferred over punctual measures.
2.3.2. Selection of measures for enhancing sustainable mobility
- Assessment of the need to apply such measures to the specific intervention road site. This should be based on: (1) available urban plans, such as Sustainable Urban Mobility Plans (SUMPs, see [53]), (2) the function of the road on which the intervention is planned within the relevant urban network, and (3) the boundary conditions, such as traffic volumes, including pedestrian and cyclist flows, carriageway width, and land use [54].
- If needed, evaluation of possible infrastructure-related interventions: modify the existing horizontal alignment (i.e., through chicanes, pedestrian refuges, curb extensions, and chokers); the elevation profile (i.e., through speed bumps, humps, cushions, tables, raised pedestrian crossings, and intersections); or surface materials, texture, and color. Bike lanes/paths, alongside the related facilities, could be implemented where appropriate, if absent or inadequate.
- If needed, the evaluation of possible management-related interventions, such as restrictions on vehicle speeds, maneuvers, and access.
- Assessment of the effect of these measures on safety. Generally, these effects are varying, depending also on the application contexts, and some contrasting findings were reported indeed (see e.g., [4,55]). However, traffic calming measures, especially if implemented at the area-wide level, are effective, even if with variable outcomes. For example, reducing operating vehicular speeds down to 30 km/h leads to a consistent decrease in the fatality likelihood for pedestrians involved in crashes [56].
2.3.3. Definition of sets of countermeasures
- One/more sets of short-term safety measures that are relatively inexpensive, easily implementable, and do not require additional approvals, even if they are likely to have only a small positive impact on safety;
- One/more sets of long-term safety measures, typically consisting of drastically modifying and re-shaping the road geometry, which is relatively expensive and requires additional approvals (e.g., for expropriating lands), but is likely to have a strong impact on safety;
- Site-wide intervention composed of different measures, typically based on speed management (traffic calming), with different costs, benefits, and implementation issues.
2.4. Methods: Economic Assessment
2.4.1. Estimation of safety benefits
- SBi = Safety benefit associated to the i-th set of countermeasures (€);
- AASC = Average Crash Social Cost (€/crash);
- = expected number of crashes (see Equation (2)) before the intervention (i-th set of countermeasures) on the j-th homogeneous sub-site (sub-segment or intersection) included in the examined road site (crashes);
- = expected number of crashes (see Equation (2)) after the intervention (i-th set of countermeasures) on the j-th homogeneous sub-site (crashes);
- k-th Crash Modification Factor associated to the i-th set of countermeasures proposed for the j-th homonegeous site with respect to the original conditions (-).
2.4.2. Cost-benefit analyses
3. Results from the Case Study Application of the Integrated Framework
3.1. Results: End of Network Screening
3.1.1. Problem/Solution 3.1.A
3.1.2. Problem/Solution 3.1.B
3.2. Results: Diagnosis
3.2.1. Problem/Solution 3.2.A
- Number of lanes (and/or carriageway width) not coherent with the function and the traffic flow (e.g., wide one-way minor collectors potentially allowing overtaking, double parking, and high speeds, in contrast with the presence of a relevant flow of VRUs, which may generate dangerous conflicts in residential areas);
- Intersection type and/or dimensions not coherent with the function of the intersecting roads (e.g., signalized intersections without specialized lanes, traffic islands, dedicated signals for major collector roads intersecting);
- Several accesses/minor intersections on arterial roads (which may be differently managed);
- Absence of infrastructures and facilities for VRUs (even in case of their relevant presence).
3.2.2. Problem/Solution 3.2.B
- Arterial roads entering into the city, departing/being the continuation of rural roads (Figure 7);
- High-level roads approaching to cities/towns, such as freeways or multi-lane highways.
3.2.3. Problem/Solution 3.2.C
3.3. Results: Selection of Countermeasures
3.3.1. Problem/Solution 3.3.A
3.3.2. Problem/Solution 3.3.B
3.3.3. Problem/Solution 3.3.C
- Long-term measures for high-level urban roads, which include the site geometric reconfiguration, and not only inexpensive punctual measures.
- Short-term measures for secondary roads, having scarce traffic flow and public transports.
3.4. Results: Economic Assessment
3.4.1. Problem/Solution 3.4.A
- The models in [43] for intersections, since: (a) the intersections considered for pilot applications have no intersecting segments with notable grades, and are often not provided with dedicated left/right-turn lanes (variables considered in [74]); (b) a scarce transferability [75] of the models in [74] was determined by applying it in another Italian city; (c) they are more recent; and (d) they include traffic variables for major/minor roads in the three-leg and four-leg cases. Clearly, even detailed models [42,43] lack some safety-related variables (Table 2). Hence, the effect of interventions affecting those variables on safety performances should be estimated through additional CMFs (by carefully checking their applicability).
3.4.2. Problem/Solution 3.4.B
3.4.3. Problem/Solution 3.4.C
4. Discussion
4.1. Integration of Quantitative Assessments and Qualitative Methods and Concepts
4.2. Highlighted Problems and Proposed Solutions within the Design Framework Application
4.3. Concluding Remarks
Supplementary Materials
Author Contributions
Funding
Acknowledgements
Conflicts of Interest
References
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PART OF THE INSPECTION SHEET FOR ROAD SEGMENTS | ||||
---|---|---|---|---|
MACRO-ITEM | ITEM | PARAMETER | INDICATOR | JUDGMENT (TO BE FILLED BY THE OPERATOR) |
ROAD CARRIAGEWAY | CARRIAGEWAY | SHOULDER | ABSENCE/INSUFFICIENT WIDTH | No issues detected, light or severe issues detected |
NARROWING IN PRESENCE OF HIGH PEDESTRIAN FLOWS | No issues detected, light or severe issues detected | |||
TRAVEL LANES | INADEQUATE WIDTH | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
SPECIALIZED LANE | INADEQUATE WIDTH | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
RESERVED LANE | INADEQUATE WIDTH | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
BUS STOP | INADEQUATE DIMENSIONS | No issues detected, light or severe issues detected | ||
LOCALIZATION | No issues detected, light or severe issues detected | |||
DISCONTINUITY OF PEDESTRIAN PATHS | No issues detected, light or severe issues detected | |||
MEDIAN | ABSENCE | No issues detected, light or severe issues detected | ||
EFFECTS ON VISIBILITY | No issues detected, light or severe issues detected | |||
INADEQUATE ORGANIZATION OF AREAS | No issues detected, light or severe issues detected | |||
PARKING LOTS | INADEQUATE ORGANIZATION OF AREAS | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
PEDESTRIAN/BIKE PATHS | CROSS SECTION WIDTH | No issues detected, light or severe issues detected | ||
PAVEMENT MAINTENANCE | No issues detected, light or severe issues detected | |||
PRESENCE OF OBSTACLES | No issues detected, light or severe issues detected | |||
PRESENCE OF MEDIANS | No issues detected, light or severe issues detected | |||
PART OF THE INSPECTION SHEET FOR ROAD INTERSECTIONS | ||||
MACRO-ITEM | ITEM | PARAMETER | INDICATOR | JUDGMENT (TO BE FILLED BY THE OPERATOR) |
ROAD CARRIAGEWAY | CARRIAGEWAY | SHOULDER | ABSENCE/INSUFFICIENT WIDTH | No issues detected, light or severe issues detected |
NARROWING IN PRESENCE OF HIGH PEDESTRIAN FLOWS | No issues detected, light or severe issues detected | |||
SPECIALIZED LANE | INADEQUATE WIDTH | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
RESERVED LANE | INADEQUATE WIDTH | No issues detected, light or severe issues detected | ||
INADEQUATE COORDINATION WITH OTHER FLOWS | No issues detected, light or severe issues detected | |||
TRAFFIC ISLAND | EFFECTS ON VISIBILITY | No issues detected, light or severe issues detected | ||
INADEQUATE ORGANIZATION OF AREAS | No issues detected, light or severe issues detected | |||
PEDESTRIAN/BIKE PATHS | CROSS SECTION WIDTH | No issues detected, light or severe issues detected | ||
PAVEMENT MAINTENANCE | No issues detected, light or severe issues detected |
Segments | Intersections | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Variables | Included in Final Models as Retrieved in: | Variables | Included in Final Models as Retrieved in: | |||||||||
[42] | [73] | No Model | [43] | [74] 1 | [73] 2 | No Model | [43] | [74] 1 | [73] 2 | No Model | ||
Three-Legged Intersections | Four-Legged Intersections | |||||||||||
AADT | ✔ | ✔ | AADT of major road section | ✔ | ✔ | ✔ | ✔ (no-control, stop) | |||||
Lenght of road section | ✔ | AADT of minor road section | ✔ | ✔ (stop) | ✔ | ✔ | ||||||
Speed limit | ✔ | Total entering AADT volume in the intersection | ✔ | ✔ | ||||||||
Road width | ✔ | Lane balance | ✔ | ✔ | ||||||||
Number of accesses | ✔ | ✔ | Median presence on one leg of major road | ✔ | ||||||||
Number of minor exits | ✔ | Median presence on two legs of major road | ✔ | ✔ (stop) | ||||||||
Parking | ✔ | Median presence on two legs of minor road | ✔ | |||||||||
Land use | ✔ | Total number of entering lanes on major road | ✔ | |||||||||
One-way | ✔ | Number of lanes on minor road | ✔(signalized) | |||||||||
Number of lanes | ✔ | Average lane width on minor road | ✔ (stop) | ✔ | ✔ (no-control, signalized) | |||||||
Road signs on minor roads/accesses | ✔ | Number of one-way legs | ✔ | |||||||||
Pavement conditions | ✔ | One-way on major road | ✔ (no-control) | ✔ (no-control, signalized) | ||||||||
Road markings | ✔ | One-way on minor road | ✔ (stop) | ✔ (no-control) | ||||||||
Presence of bikelanes/paths | ✔ | Right turn presence on major road 3 | ✔ | ✔ | ✔ (stop, signalized) | |||||||
Sidewalk width | ✔ | Right turn presence on minor road 3 | ✔ (stop, signalized) | |||||||||
Median presence | ✔ | Left turn presence on major road 3 | ✔(no-control) | ✔ (stop) | ||||||||
Bus stops | ✔ | Left turn presence on minor road 3 | ✔ (stop, signalized) | |||||||||
Bus–taxi lane | ✔ | Sidewalk width on major road | ✔ (stop) | ✔ (signalized) | ||||||||
Sidewalk width on minor road | ✔ (no-control) | |||||||||||
Grade on major road section 4 | ✔ (no-control) | ✔ (stop, signalized) | ||||||||||
Grade on minorroad section 4 | ✔ | ✔ (no-control, stop) | ||||||||||
Road markings | ✔ | |||||||||||
Phasing of signals | ✔ (signalized) | |||||||||||
Sight distance | ✔ | ✔ | ||||||||||
Pavement conditions | ✔ | ✔ | ||||||||||
Presence of bike lanes/paths | ✔ | ✔ | ||||||||||
Bus–taxi lane | ✔ | ✔ |
Set A (Short-Term Measures) | Set B (Short-Term Measures) | Set C (Long-Term Measures) | Set D (Long-Term Measures + Sustainable Mobility Infrastructures) | |||
---|---|---|---|---|---|---|
1. Installation of transverse rumble strips across lanes in approach at intersections 2. Installation of automatic speed control along segments | 1. Replacement of the friction course 2. Improvement of both road markings and signs 3. Implementation of new pedestrian crossings 4. Installation of optical speed bars 5. Installation of pedestrian crosswalk countdowns 6. Installation of intersection flashing warning signs 7. Turning on traffic lights at night 8. Pruning vegetation and trees | 1. Reconfiguration of the road section along all segments 2. Realization of a chicane at bus stops 3. Implementation of pedestrian crossings 4. Implementation of islands for pedestrians 5. Implementation of traffic islands 6. Installation of barriers along the sidewalks near the school | 1. Implementation of 2 roundabouts in place of existing four-leg intersections 2. Implementation of a bi-directional bike lane 3. Converting pedestrian crossings into bike–pedestrian crossings 4. Replacing lighting systems with light-emitting diode (LED) lights 5. Remove trees from critical places | |||
Set | Combination | Costs (€) | Benefits (€) | NPV (Net Present Value) | CBA | Incremental CBA |
A | 1 | Third | Fifth | Fifth | Second | Fourth |
B | 2 | Fourth | Fourth | Fourth | Sixth | Third |
A+B | 3 | Fifth (more expensive) | First (more benefits) | First1 | Fifth | First1 |
C | 4 | First (less expensive) | Sixth | Sixth | First1 | Fifth |
D | 5 | Second | Third | Third | Fourth | Second |
D 2 | 6 | Second | Second | Second | Third | Second |
Segments | Intersections | |||
---|---|---|---|---|
Variables | Included in Final Models as Retrieved in: | Variables | Included in Final Models as Retrieved in: | |
[76] 1 | [77] | [55] 2 | ||
All | Unsignalized | |||
Vehicle flow | ✔ | Vehicle flow | ✔ | ✔ |
Cycling flow | ✔ | Cycling flow | ✔ | ✔ |
Land use | ✔ | Two-way/one-way cycle track | ✔ ** | |
Function of street | ✔ | Distance between bike lane and carriageway | ✔ ** | |
Visibility | ✔ | Red color and quality of markings for bicycle crossings | ✔ ** | |
Bike transit prohibition | ✔ * | Dedicated VRU crossing | ✔ ** | |
Raised bicycle crossing or other speed reducing measures | ✔ ** |
© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
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Colonna, P.; Intini, P.; Berloco, N.; Fedele, V.; Masi, G.; Ranieri, V. An Integrated Design Framework for Safety Interventions on Existing Urban Roads—Development and Case Study Application. Safety 2019, 5, 13. https://doi.org/10.3390/safety5010013
Colonna P, Intini P, Berloco N, Fedele V, Masi G, Ranieri V. An Integrated Design Framework for Safety Interventions on Existing Urban Roads—Development and Case Study Application. Safety. 2019; 5(1):13. https://doi.org/10.3390/safety5010013
Chicago/Turabian StyleColonna, Pasquale, Paolo Intini, Nicola Berloco, Veronica Fedele, Giuseppe Masi, and Vittorio Ranieri. 2019. "An Integrated Design Framework for Safety Interventions on Existing Urban Roads—Development and Case Study Application" Safety 5, no. 1: 13. https://doi.org/10.3390/safety5010013
APA StyleColonna, P., Intini, P., Berloco, N., Fedele, V., Masi, G., & Ranieri, V. (2019). An Integrated Design Framework for Safety Interventions on Existing Urban Roads—Development and Case Study Application. Safety, 5(1), 13. https://doi.org/10.3390/safety5010013