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