Place Value Wiki - B3. Street design and safety from collisions

B3. Street design and safety from collisions

Street liveability and safety

Aims and method:

Explores the relationship between conventional approaches to highway design that use safety arguments to discourage the use of aesthetic roadside treatments (e.g. street trees) and attempts to create a more liveable built environment. Reviews evidence from a variety of mainly American empirical studies.

Key findings:

· There is little evidence to support the claim that liveable streetscape treatments are less safe than their more conventional counterparts, and the weight of the evidence suggests that they can possibly enhance a roadway’s safety performance

· The more basic problem appears to be that safety and liveability objectives are often in direct conflict with the overarching objective of mobility, and its proxy—speed

· Design approaches are too often based on a philosophy that discounts the important relationship between driver behaviour and safety, namely that drivers will modify their behaviour (and slow down) when faced with a potential hazard.

Reference:

Dumbaugh, E., & Gattis, J. (2005) Safe Streets, Livable Streets, Journal of the American Planning Association, 71(3), 283-300

http://www.tandfonline.com/doi/pdf/10.1080/01944360508976699?needAccess=true

Street design and collision incidences

Aims and method:

Examines the relationship between community design and crash incidence. The study analyses GIS data on crash incidence and urban form from the City of San Antonio (USA) using negative binomial models.

Key findings:

· Many of the safety assumptions embedded in contemporary community design practice are not substantiated by the empirical evidence.

· While disconnecting local street networks and relocating non-residential uses to arterial thoroughfares can reduce neighbourhood traffic volumes in residential areas, this does not improve safety, but substitutes one set of safety problems for another.

· Urban arterials, arterial-oriented commercial developments, and big box stores are associated with increased incidences of traffic-related crashes and injuries

· Higher-density communities with more traditional, pedestrian-scaled retail configurations are associated with fewer crashes.

Reference:

Dumbaugh, E., & Rae, R. (2009). Safe urban form: revisiting the relationship between community design and traffic safety. Journal of the American Planning Association, 75(3), 309-329.

http://www.tandfonline.com/doi/abs/10.1080/01944360902950349

Urban sprawl and pedestrian fatalities

Aims and method:

Sought to determine the association between urban sprawl and traffic fatalities. A sprawl index was created by applying principal components analysis to data for 448 US counties in the largest 101 metropolitan areas. Regression analysis was used to determine associations between the index and traffic fatalities

Key findings:

· For every 1% increase in the index (i.e., more compact, less sprawl), all-mode traffic fatality rates fell by 1.49% (P < .001) and pedestrian fatality rates fell by 1.47% to 3.56%, after adjustment for pedestrian exposure (P < .001).

· Urban sprawl was directly related to traffic fatalities and pedestrian fatalities.

Reference:

Ewing, R., Scheiber, R & Zeeger, C. (2003). Urban Sprawl as a Risk Factor in Motor Vehicle Occupant and Pedestrian Fatalities, American Journal of Public Health, 93(9), 1541-1545.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1448007/

Road design and safety

Aims and method:

Summarises the literature on the relationship between the built environment and traffic safety. A wide ranging review covers both macro and micro scale design factors.

Key findings:

· The traffic environments of dense urban areas appear to be safer than the lower-volume environments of the suburbs.

· Fewer miles are driven on a per capita basis, and the driving that is done is at lower speeds that are less likely to produce fatal crashes.

· In dense urban areas, less-‘forgiving’ design treatments—such as narrow lanes, traffic-calming measures, and street trees close to the roadway—appear to enhance a roadway’s safety performance when compared to more conventional roadway designs.

· The reason for this apparent anomaly may be that less-forgiving designs provide drivers with clear information on safe and appropriate operating speeds.

Reference:

Ewing, R., & Dumbaugh, E. (2009). The Built Environment and Traffic Safety A Review of Empirical Evidence, Journal of Planning Literature, 23(4), 347-367.

http://reconnectinglee.com/wp-content/uploads/2015/08/Built-Environment-and-Traffic-Safety-Review-of-Empirical-Evidence.pdf

Street pattern and crash severity

Aims and method:

Examines the effect of different street patterns on crash severity using the City of Calgary as a case study. In this study, street pattern is classified into four categories: grid-iron, warped parallel, loops and lollipops, and mixed patterns. Their effects on injury risk are examined together with other factors including road features, drivers’ characteristics, crash characteristics, environmental conditions and vehicle attributes. Pedestrian and bicycle crash data for the years 2003–2005 were utilised to develop a multinomial logit model of crash severity.

Key findings:

· Relative to grid-iron and other street patterns, loops and lollipop designs are associated with a higher likelihood of non-fatal injury in the event of a collision between a motor vehicle and a pedestrian or cyclist and a lower likelihood of non-injury or fatal injury

· Loops and lollipops design tends to have a stronger traffic calming effect. If a crash occurs in this situation, it reduces the fatality risk because of less impact force to the bicyclists or pedestrian. But loops and lollipops also reduce sight distances which increases impact speed which is significant in enhancing the probability of injury of pedestrians and cyclists.

· A crash was likely to be more severe if it occurred on divided roads with barriers

Reference:

Mohammad Rifaat, S., Tay, R & de Barros, A. (2010). Effect of street pattern on the severity of crashes involving vulnerable road users, Accident Analysis and Prevention 43(1), 276-283.

http://www.sciencedirect.com/science/article/pii/S0001457510002575

Street network design and safety

Aims and method:

Assesses the effect of street and street network characteristics on total crashes, severe injury crashes, and fatal crashes. Negative binomial regression models were used with data from over 230,000 crashes taking place over 11 years in 24 California cities was. The research controlled for variables such as vehicle volumes, income levels, and proximity to limited access highways and to the downtown area.

Key findings:

· For all levels of crash severity, street network characteristics correlate with road safety outcomes.

· Denser street networks with higher intersection counts per area are associated with fewer crashes across all severity levels (and with higher levels of walking).

· Conversely, increased street connectivity (to traffic) as well as additional travel lanes along the major streets correlated with more crashes.

Reference:

Marshall, WE. & Garrick, N. (2011). Does street network design affect traffic safety?, Accident Analysis and Prevention 43(3), 769-781.

http://www.sciencedirect.com/science/article/pii/S0001457510003179

Shared space, impact and use

Aims and method:

Evaluates the impact of shared spaces schemes on users, street use and the street environment. A broad range of available evidence on the use of street spaces was collated and reviewed as the first stage in building a more robust evidence base.

Key findings:

· Well-designed schemes in appropriate settings can bring benefits in terms of visual amenity, economic performance and perceptions of personal safety.

· There is no evident safety benefit or dis-benefit, casualty numbers tend to stay relatively constant, although they are generally low prior to scheme implementation.

· The limited data available on user flow suggests that the relatively constant casualty numbers may be in the context of significant increases in the flows of pedestrians and cyclists, suggesting a reduction in risk.

· There is some evidence from Dutch research that casualty numbers may be greater in shared space schemes with high vehicle flows compared to traditional layouts.

· Surveys of users tend to indicate that perceptions of a street improve when shared space is implemented. Level surfaces in shared space can create specific difficulties for some visually impaired people.

· Shared Space schemes featuring a level surface seem to be most acceptable to people of all abilities when a clearly defined part of the space is free from motorised vehicles.

Reference:

MVA Consultancy (2009). DfT Shared Space Project, Stage 1: Appraisal of Shared Spaces

http://webarchive.nationalarchives.gov.uk/20091204120415/http://www.dft.gov.uk/pgr/sustainable/sharedspace/stage1/pdf/stage1.pdf

Shared spaces in operation

Aims and method:

Provides an operational assessment of Shared Space schemes in the UK. Data was collected at ten sites taking into account; street type (link, square or junction), the level of traffic and pedestrian flows (high, medium and low) and various street characteristics. Observational data of pedestrian and driver behaviour was subjected to statistical analysis.

Key findings:

· Design characteristics intended to reduce the demarcation and physical barrier between the footway and carriageway do achieve a higher percentage of pedestrians using the carriageway then might otherwise be expected. Conversely, higher traffic flows are likely to discourage pedestrians using the carriageway.

· Higher pedestrian volumes as well as reducing the demarcation between the footway and carriageway areas are likely to decrease the traffic speed.

· Drivers and pedestrians were found to equally give way to each other at fully Shared Space sites. The factors that appeared to encourage the driver to give way in a pedestrian- vehicle encounter included; lower vehicle speeds and flows, reducing the demarcation between the footway and the carriageway as well as encouraging pedestrians to use the carriageway.

· Drivers were fourteen times more likely to give way to pedestrians if pedestrians were present in the carriageway.

Reference:

MVA Consultancy (2010). Designing the Future, Shared Space: Operational Assessment

https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/3886/ltn-1-11-quantitative.pdf

Shared space and user perceptions

Aims and method:

Examines the actual experience of different users using shared space schemes in the UK. qualitative research consisted of accompanied journeys and interviews with a number of different user types: drivers; non-disabled pedestrians; visually impaired pedestrians; mobility impaired pedestrians; pedestrians with learning difficulties; and pedestrians who are deaf or hard of hearing

Key findings:

· In all street designs there are commonalities among different user types including a preference for clearly defined areas for vehicles and pedestrians and designated crossing points.

· The majority of pedestrian participants (both disabled and non-disabled) preferred wide pavements, narrow carriageways with one way traffic, and reduced vehicle flow and vehicle speeds

· Drivers tended to prefer clear rules/guidance and for the behaviour of all users (both pedestrians and other vehicles) to be predictable.

· The presence of kerbs and clearly defined pavements and roadways is well understood by all user types and pedestrians prefer not to have to concentrate on, and therefore interact with other users, during their journey.

· In aggregate the experience of shared streets does not seem to be a positive one.

Reference:

MVA Consultancy (2010). Designing the Future, Shared Space: Qualitative Research

https://nacto.org/docs/usdg/shared_space_qualitative_research_dickens.pdf

Local Variations in the Impacts of Built Environments on Traffic Safety

Aims and method:

Studies the influence of different built environment configurations on the severity of injuries incurred in traffic incidents. Case studies are selected from a range of uniquely configured neighbourhoods in the Austin area of Texas (USA). Geographically Weighted Regression and Binomial Regression is used to test for relationships between the severity of injuries, and the density, design and diversity of each neighbourhood.

Key findings:

Roads able to accommodate higher speeds produce more traffic fatalities with the likelihood of accidents increasing in downtown areas; this trend is attributed to the mixture of traffic types, including non-motorised modes of transport.
Commercial and office areas experience more occurrences of injury at more significant occurrence rates than downtown locations; this attributed to commercial areas attractinghigher traffic volumes and being located along arterial roads with stores located behind parking lots, increasing the likelihood of pedestrian-vehicle collisions.

Reference:

Yu, C. and Xu, M. (2017). Local Variations in the Impacts of Built Environments on Traffic Safety. Journal of Planning Education and Research, 38(3), 314 - 328

https://journals.sagepub.com/doi/full/10.1177/0739456X17696035

Road safety for elderly users

Aims and method:

The study applied a negative binomial regression to a set of built environmental variables to study the occurrence of accidents involving elderly and younger (non-elderly) pedestrians in Madrid (Spain) between 2006 and 2018. The model considers a selection of built environmental factors per city district, linked to land use, infrastructure, and socioeconomic indicators.

Key findings:

· Elderly pedestrian–vehicular collisions could be avoided with the existence of wider pavements and a greater density of traffic lights.

· Unlike younger pedestrian accidents, these accidents occur more in districts with a higher ageing population percentage and higher traffic flows.

Reference:

Gálvez-Pérez, D., Guirao, B., Ortuño, A., & Picado-Santos, L. (2022). The Influence of Built Environment Factors on Elderly Pedestrian Road Safety in Cities: The Experience of Madrid. International Journal of Environmental Research and Public Health, 19(4), 2280.

https://doi.org/10.3390/ijerph19042280

Influence of streetscape environmental characteristics on pedestrian crashes at intersections

Aims and method:

The study examined the relationship between streetscape features and road traffic accidents. It extracted streetscape environment characteristics from street view images using a combination of semantic segmentation and object detection deep learning networks. These characteristics were then incorporated into the eXtreme Gradient Boosting (XGBoost) algorithm, along with a set of control variables, to model the occurrence of pedestrian crashes at intersections. Subsequently, the SHapley Additive exPlanations (SHAP) method was integrated with XGBoost to establish an interpretable framework for exploring the association between pedestrian crash occurrence and the surrounding streetscape built environment.

Key findings:

· The design of the streetscape environment significantly impacts pedestrian crash occurrences.

· From a global perspective, traffic volume and commercial land use presence are the most significant factors impacting pedestrian–vehicle collisions at intersections

· Road (wide road surfaces in urban areas indicative of dense transportation), person, and vehicle elements (a high volume of pedestrians and vehicles) extracted from the street view images are associated with higher risks of pedestrian crash onset.

· Dense tree and plant cover, reducing visibility, increase accidents, while grasses areas, affording visibility, lead to a reduced probability of pedestrian crashes.

Reference:

Yue, H. (2024). Investigating the influence of streetscape environmental characteristics on pedestrian crashes at intersections using street view images and explainable machine learning. Accident Analysis and Prevention, 205, Article 107693.

https://doi.org/10.1016/j.aap.2024.107693

Effect of street network design on traffic congestion and traffic safety

Aims and method:

The study examined the effects of street network design on congestion levels and crash rates in neighbourhoods across Utah's Wasatch Front. Propensity score matching was used to select pairwise neighbourhood samples that have other similar characteristics but differ greatly in street network design.

Key findings:

· Denser and more connected neighbourhoods are associated with lower congestion levels.

· Denser and more connected network designs were not significantly related to crash rates.

Reference:

Choi, D., & Ewing, R. (2021). Effect of street network design on traffic congestion and traffic safety. Journal of Transport Geography, 96, Article 103200.

https://doi.org/10.1016/j.jtrangeo.2021.103200

Pedestrian crossing design efficiency and safety

Aims and method:

The study identified three pedestrian crossing patterns and analysed them from the perspective of efficiency and safety. For efficiency, delay models were proposed by considering diagonal pedestrian movements. For safety, exposure conflicts and the number of potential traffic accidents were analysed. Then delay and potential accidents were converted into money value respectively and the total cost was calculated.

Key findings:

· Symmetric intersections perform better in terms of increasing capacity and decreasing pedestrian delay compared with conventional intersections.

· The key is removing drivers’ confusion and maintaining good traffic order.

Reference:

Tang, L., Liu, Y., Li, J., Qi, R., Zheng, S., Chen, B., & Yang, H. (2020). Pedestrian crossing design and analysis for symmetric intersections: Efficiency and safety. Transportation Research. Part A, Policy and Practice, 142, 187–206.

https://doi.org/10.1016/j.tra.2020.10.012

Factors impacting bike crash severity in urban areas

Aims and method:

The study examined the critical factors that influence severe bicycle crash outcomes to identify and prioritise policies and actions to mitigate these risks. The study approach involved the use of classification models (logistic regression, decision tree and random forest), as well as techniques for treating unbalanced data by under sampling, oversampling, and weighted cost sensitivity (CS) learning, applied to bike crash data from the State of Tennessee’s two largest urban areas, Nashville and Memphis.

Key findings:

· Key factors that most influence cyclist safety outcomes are issues related to lighting (placement of street lighting along popular bike routes) followed by roadway type (separating bicycle lanes from traffic and, where this is not possible, creating sufficient street width for an on-street bike lane)

· Risk mitigation strategies aimed at these factors merit the most serious consideration, including reviewing all streets with speed limits above 30 mph to assess whether they can be lowered.

Reference:

Dash, I., Abkowitz, M., & Philip, C. (2022). Factors impacting bike crash severity in urban areas. Journal of Safety Research, 83, 128–138.

https://doi.org/10.1016/j.jsr.2022.08.010

Effect of urban street trees on pedestrian safety

Aims and method:

In this study, a micro-level frequency model was established to evaluate the effects of tree density and tree canopy cover on pedestrian injuries, accounting for pedestrian crash exposure based on comprehensive pedestrian count data from Melbourne, Australia. In addition, effects of road geometry, traffic characteristics, and temporal distribution were considered.

Key findings:

· Pedestrian injury decreases with tree density and tree canopy cover (trees seeming to act as a psychological indicator to moderate speed) and where there are pedestrian crosswalks.

· Pedestrian injury increases with road width, bus stop, on-street parking, and speed.

Reference:

Zhu, M., Sze, N. N., & Newnam, S. (2022). Effect of urban street trees on pedestrian safety: A micro-level pedestrian casualty model using multivariate Bayesian spatial approach. Accident Analysis and Prevention, 176, Article 106818.

https://doi.org/10.1016/j.aap.2022.106818

Impact of urban design and the built environment on road traffic crashes

Aims and method:

The study identified urban design factors that can influence road traffic crashes. The factors are categorised under the dimensions of land use, street furniture, buildings, and landscape. Road crashes’ data was collected from 1122 Emergency Services of Punjab, Pakistan, while data regarding built environment factors were collected using field surveys. Kernel density estimation technique was used to identify crash hotspots. Multiple and Tobit regression models were used to determine the impact of these indicators on road traffic crashes.

Key findings:

· Most collisions were along the main highways.

· Land use type can affect road accidents. Commercial, residential, and recreational land uses positively influenced traffic crashes.

· Informal stops, poor traffic sign visibility and road conditions, footpath/setback encroachments, and green medians alongside roads all negatively affected accident density.

Reference:

Umair, M., Rana, I. A., & Lodhi, R. H. (2022). The impact of urban design and the built environment on road traffic crashes: A case study of Rawalpindi, Pakistan. Case Studies on Transport Policy, 10(1), 417–426.

https://doi.org/10.1016/j.cstp.2022.01.002

Effectiveness of 20 mph zones and 20 mph limits

Aims and method:

The research investigates the effect of 20 mph speed ‘zones’ and ‘limits’ on a range of health outcomes and seeks to establish if there are differences in the effectiveness of 20 mph zones and 20 mph limits. MEDLINE, EMBASE, Web of Science and Transport Research Information Service (TRIS) databases were searched [1983–January 2019) to identify relevant studies. Reference lists, relevant systematic reviews and the grey literature were also searched.

Key findings:

· 20 mph ‘zones’ are effective in reducing collisions and casualties.

· There is not sufficient evidence on the effect of 20 mph ‘zones’ on pollution, inequalities or liveability.

Reference:

Cleland, C. L., McComb, K., Kee, F., Jepson, R., Kelly, M. P., Milton, K., Nightingale, G., Kelly, P., Baker, G., Craig, N., Williams, A. J., & Hunter, R. F. (2020). Effects of 20 mph interventions on a range of public health outcomes: A meta-narrative evidence synthesis. Journal of Transport & Health, 17, 100633.

https://doi.org/10.1016/j.jth.2019.100633

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