Effect of street pattern on the severity of crashes involving vulnerable road users

Abstract

Road crashes not only claim lives and inflict injuries but also create an economic burden to the society due to loss of productivity. Although numerous studies have been conducted to examine a multitude of factors contributing to the frequency and severity of crashes, very few studies have examined the influence of street pattern at a community level. This study examined 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 utilized to develop a multinomial logit model of crash severity. Our results showed that compared to other street patterns, loops and lollipops design increases the probability of an injury but reduces the probability of fatality and property-damage-only in an event of a crash.

Introduction

Road crashes are a leading cause of deaths and injuries in many developed and developing countries. Around the world, about 1.2 million people are killed on the roads each year (WHO, 2004). In Canada, for example, about 3000 road users are killed and 200,000 are injured while traveling on our road networks (Transport Canada, 2004). The province of Alberta alone experiences over 120,000 motor vehicle collisions every year, which result in the deaths of about 400 road users (AIT, 2006). In addition to this staggering number, traffic fatalities have consistently been the leading cause of death for young Albertan (Capital Health, 2007) and Canadian adults (Statistics Canada, 2005).

Among the different road user groups, pedestrians and bicyclists are the most vulnerable and often experience serious injuries due to their lack of protection. Recognizing their vulnerability, many researchers have paid special attention to pedestrian and bicycle involved crashes in order to reduce their frequency and severity (Abdel-Aty et al., 2007, Eluru et al., 2008, Sullivan and Flannagan, 2002, Noland and Quddus, 2004, Macpherson et al., 2004, J.-K. Kim et al., 2007) The injury and fatality risks of these vulnerable road users in traffic crashes are influenced by a multitude of factors, including vehicle characteristics, roadway design characteristics, driver behaviours, types of collisions and environmental conditions. Since evidence-based safety improvements can only be successfully applied if the relevant factors that contribute towards increasing the severity of these crashes are known, many studies have been conducted to identify and quantify the impacts of these contributing factors.

For example, Eluru et al. (2008) found that the general pattern and relative magnitude of elasticity effects of injury severity determinants were similar for pedestrians and bicyclists. Their analysis also suggests that the most important variables influencing non-motorist injury severity are the age of the individual (the elderly are more injury-prone), the speed limit on the roadway (higher speed limits lead to higher injury severity levels), location of crashes (those at signalized intersections are less severe than those elsewhere), and time-of-day (darker periods lead to higher injury severity). In another study, Abdel-Aty et al. (2007) found that the age and gender of the pedestrian or cyclist, proximity to elementary school, number of lanes and speed limit had a significant effect on frequency of crashes involving non-motorists, especially school aged children.

In addition to studies that examined factors contributing to both pedestrian and cyclist crashes, many studies have also been conducted to identify factors contributing to either pedestrian or cyclist crashes. For example, J.-K. Kim et al. (2007) found that greater vehicle speeds prior to impact, truck involved accidents, speeding-involved accidents, intoxicated driver or bicyclist, bicyclist aged 55 and over, inclement weather, darkness without streetlights and head-on collision had a significant effect of the severity of bicycle involved crashes. In another study, Lapparent (2005) found that wearing of helmet and the age of cyclist had an impact on the severity of crashes involving cyclists. Also, Macpherson et al. (2004) found that children living outside urban centres had an increased risk of hospitalisation due to bicycling-related injuries.

With respect to pedestrian crash severity, Ivan et al. (2001) found that vehicle type, driver alcohol involvement, pedestrian alcohol involvement, and pedestrian aged over 65 significantly increased the injury severity of pedestrians. Sullivan and Flannagan (2002) found that pedestrian fatalities were three to four times more likely at night than in daylight and seven times higher in unlighted intersections than in lighted intersections. Noland and Quddus (2004) found that more severe pedestrian injuries were associated with areas of lower income population, higher percentage of local roads, higher per capita expenditure on alcohol, and larger numbers of people.

Although the effects of numerous factors have been explored in past studies, little research has been done to explore the effect of street pattern on crash severity. Among the commonly observed street pattern, grid-iron is the more traditional street pattern and is comprised of intersecting streets that are mostly straight thoroughfares. On the other end is the loops and lollipops design which is comprised of roads that are usually curvilinear. This limited access street pattern with loops and cul-de-sacs are often known as the loops and lollipops design. Over the last fifty years, the loops and lollipops design has become the basic building block of many suburban road networks in most North American cities. Although developed to improve the social living environment, this combination of cul-de-sacs and loop streets has the support of many traffic engineers because of its traffic calming effects.

Perhaps due to its intuitive appeal, few studies were conducted to examine the impact of this design on road crashes. In one of the few early studies on street pattern and traffic safety, Marks (1957) found that 50% of all intersections in Los Angeles with the traditional grid pattern had at least one collision whereas only 8.8% of the intersections in limited access pattern had collisions between 1951 and 1956. However, the results of this study have to be interpreted with caution in today's context because it examine only right angled crashes at intersections and ignore all non-intersection crashes. In addition, it does not control for many important influences such as land use and neighbourhood density as well as the economic, social and demographic composition.

In a recent study, Rifaat and Tay (2009) explored the effect of street pattern on the injury risk in two-vehicle crashes using data from the City of Calgary. They classified street pattern or urban form into four categories: grid-iron, warped parallel, loops and lollipops, and mixed patterns. The effects of street pattern on injury risk were examined using a logistic regression model that included control variables like road features, driver characteristics, crash characteristics, environment conditions and vehicle attributes. Their study found that, compared with grid-iron, loops and lollipops type design decreased the injury risk of crashes involving two-vehicle. Although insightful, this study examined only the factors contributing to the injury risk of vehicle–vehicle crashes and not crashes involving vulnerable road users.

In another study, Rifaat et al. (2009) examined the effect of street pattern on the number crashes using similar data from the City of Calgary. They found that currently popular road patterns such as warped parallel, loops and lollipops, and mixed shapes were associated with fewer crashes than traditional gridiron pattern. However, their study examined only total crashes and ignored the severity of crashes. To improve traffic safety, we need to identify and address the factors contributing to both the frequency and severity of crashes.

The above studies are noteworthy because they focused on local roads in general and the street patterns in different community areas in particular. Although most local roads have low traffic volume and speed, our data showed that a little more than half (50.35%) of the total 106,412 crashes occurred on local and collectors roads in Calgary during the period 2003–2005 and the orientation of these local and collector roads might have some influences on community crashes and crash related severities which cannot be ignored from traffic safety perspective. For example, of the 53,574 crashes in the 227 community areas in Calgary, 16.14% have been in community areas with a predominantly gridiron street pattern, 17.21% in warped parallel areas, 53.73% in loops and lollipops areas and 12.92% in areas with mixed street patterns.

One important issue that has not yet been discussed in the literature regarding the loops and lollipops design is the unintended consequences due to risk compensation behaviour, especially among vulnerable road users. In many modern suburban neighbourhoods, especially in the cul-de-sacs, parents and caregivers are more willing to let children play on the roads (Veitch et al., 2006). This risk compensation behaviour may result in more crashes involving children and since these road users are more vulnerable, these crashes are likely to be more severe. In addition, loops and lollipops designs have horizontal alignment issues and restricted sight distances because of parked vehicles, trees and other roadside furniture. These restrictions may increase the severity of a crash involving pedestrians and cyclists due to shorter stopping distances and higher impact speeds. Therefore, the effect of street pattern on injury risks in a crash is not as certain as some may believed.

Accordingly, the main objective of this study is to understand how different street patterns affect the severity of pedestrian and bicycle involved motor vehicle collisions. Besides street pattern, other factors related to road features, drivers’ characteristics, crash characteristics, environmental condition and vehicle attributes are also explored. The City of Calgary is chosen as a case study because this city is growing rapidly due to the economic boom in the oil and gas sector. To accommodate the increased population, the city is expanding laterally and many new communities are being built. Therefore, evidence on the effects of different neighbourhood designs and street patterns on traffic safety is needed to help policy makers, developers and residents make more informed choices.