Traffic Accidents and Geometry Elements of Road Design

Introduction

A lot has been said about road safety tips and guidelines. However, road geometry factors that cause road accidents have not received as much attention and coverage. People simply do not want road accidents to occur because of their harmful consequences. Road accident investigators claim that accidents in Zimbabwe are caused by vehicle defects, over speeding, obstructions in the drivers’ way and bad roads. Although the Traffic Safety Council of Zimbabwe says most road accidents are as a result of human error, this paper saves to understand the contribution of road design geometric elements on road traffic accidents on the first Highway entry point of Gweru town from Shurugwi, in Zimbabwe. It should be understood very well that road traffic accidents can be largely prevented.

Accidents are caused by a number of factors but in this case a deep assessment is done that relates traffic accidents to four parameters of road design that are; Road alignment, Road Profile, Road Cross section and Sight distance. Since long ago, a lot of studies have been undertaken about accidents, the condition of their occurrence and also the magnitude of the damage (Aftabi Hossein and Arabani, 2012). The effects of different variables on traffic accidents obtained in these studies were for prevention or to reduce the accident rates. A negative binomial distribution was used to analyse the effect of median width on accident rate 1993. The findings proved that accident rates decreased with increasing median width (Aftabi Hossein and Arabani, 2012). It was noticed that there is actually a strong relation between accident occurrence and geometric characteristics of urban freeway sections in 1995. Miaou (1994) did a study that gave a relationship between accidents along a Highway and Highway geomatics using negative binomial regression.

In the research, he evaluated the performance of the Poisson regression, negative binomial regression and zero-inflated Poisson regression. As the first step in developing a comparison model, Miaou’s work suggested that the Poisson regression model is the best to be used in determining the relationship between Highway geometrics and accidents. Persaud presented one of the earliest studies for carrying out separate analyses for curves and tangents. The dependent variable was accident frequency, while the independent variable was road geometry. For curves, accident frequency was found to increase with section length and curvature. For tangents, the number of accidents increases with increase in section length (Persaud et al). Madziva (2018) did a research on the conformity of the geometric designs of traffic circles in Gweru to the standards of the Ministry of Transport and their contribution to truck rollovers on traffic circles.

In conclusion he found out that the design characteristics are similar to the ones used by the Ministry of Transport even though alterations were made out of the handbook guidelines due to some factors such as cost of construction and use of the road under which the roundabout was to be constructed. The above studies show that there are several variables that affect accidents and their attention points have concentrated on rural roads and there is little attention to the accidents of urban highways. Madziva (2018) concentrated his research on traffic circles overlooking the characteristics of highway designs of road sections near the traffic circles that also may contribute to road accidents. This paper surveys accidents on the Shurugwi highway entry point to Gweru town. Identifying more important variables that increase traffic accident rates in urban highways is the objective of the paper for the costs of accidents will be decreased after identifying each variable on the basis of available data and the amount of their effects.

Surveying the above studies shows that more important causes of traffic accidents in highways are traffic and geometric design parameters. According to experience of these studies, researchers tried to show simpler models with a higher probability ratio and they applied many linear and nonlinear models to present many logical models. In this research, nonlinear models are applied for modelling because of their greater compatibility with estimations on collected data. Therefore logic analysis is made by accident data recorded on the Shurugwi Highway route A18 at the Gweru entry point in Daylesford residential area Zimbabwe, during 2016 and 2017. The aim is to identify more effective variables on traffic accident occurrence in the study area and suggest scientific solutions to eradicate or reduce the rate of traffic accidents.

Justification

A series of traffic accidents happen every year at the urban entry point of Shurugwi highway to Gweru according to the traffic council of Zimbabwe report of 2014. This results in financial losses as motor vehicles become damaged and mostly the loss of human lives. Immediate action must be taken to identify the causes of these events and a scientific way to approach this problem is to consider geometric elements of the road design such as super elevation and lane width as some of the factors that contribute to the occurrence of accidents. Road crashes kill an average of four people daily in Zimbabwe. In 2014 the number of road crashes increased by 16% relative to 2013 (National Road Safety Commission, 2014). The cost of this research is small as related to the importance of the study and discovery of possible solutions to reduce the rate of accident occurrence.

Problem Statement

Even though most of road accidents are concluded to be a result of human error, a certain part of responsibility is credited to poor road structures (The Herald14). Every year a number of motorists are involved in deadly traffic accidents on the A18 (Shurugwi highway) Gweru entry point. One of the major physical problems is the shot sight distance when approaching the roundabout and limited road informative signage. At this section of the road, most accidents happen to traffic drivers without the influence of external motorists.

Research Question· What is the relationship between traffic accidents and road geometry design elements and the Shurugwi highway, Gweru town entry point section?

Specific Research Questions

• What is the relationship between road traffic accidents and elements of road geometry design?
• What factors influence horizontal road curvature designs near roundabouts and roundabout designs?
• What are the standards of sight distance, pavement width, shoulder length and superelevation of highway points toward a roundabout?

Research Objectives

• To assess the relationship between road geometric design elements and traffic accidents.

Specific Objectives

• To determine the relationship between road traffic accidents and elements of road geometry design
• To evaluate the geometric structure of the curve and its relationship to the roundabout relative to the standards of roundabout designs
• To evaluate existing geometric design elements and suggest corrective measures

Research Methodology

The aim of the research is to identify important geometric variables that must be altered to design an effective road section that aims to reduce accidents at the study site. Specific data analysis tools will be identified that will be used to integrate the research outcomes. Persaud presented one of the earliest studies for carrying out separate analyses for curves and tangents. The dependent variable was accident frequency, while the independent variables were traffic flow and road geometry. For curves, accident frequency was found to increase with section length and curvature. Data for the research is to be compiled from police reports, road profile and alignment diagrams from the department of roads and field data collection for the specific research site.

Research Outline

The paper shall consist of five independent chapters which are briefly described below:

Introduction;

This contains the background of the research putting hard emphasis on one of the major causes of road traffic accidents being the geometric parameters of road designs and brings out the problem definition and its objectives.Literature review; It gives an overview of the literature related to this research highlighting some important attempts that have been done to reduce the problem of traffic accidents by the analysis of road designs from other countries.

Methodology;

The chapter displays the sequential order to be followed from the collection of road accident data from the police accident reports and road design parameters from the department of roads to the integration of the data and mathematical models that compares accident type, related road design element and adjustments to the road geometry that can be done to reduce fatality in an event of a traffic accident.

Results and analysis;

This part discusses the results of the project as well as mathematical analysis to be performed to validate the conclusion to whether geometric elements of road design affect traffic safety and helps to check whether the objectives of the research have been attended to.

Conclusion and recommendations;

Involves presentation of the information gathered about the geometry of road design that contribute to traffic accidents and suggestive measures that can be taken to reduce the occurrence of accidents at the study site.

Area of case study

The Gweru-Shurugwi Highway commonly known as the Shurugwi road joins the City of Gweru and Shurugwi town with a paved tared road that was designed years before the independents of the country. The highway stretches a distance of 34.3km south east of Gweru to Shurugwi. The entry point of the highway to Gweru is located between Senga and Dellsford residential areas and it is marked by a series of horizontal curves and a three leg traffic circle.

Introduction

The geometric design of roads is the branch of highway engineering concerned with the positioning of the physical elements of the roadway according to standards and constraints. The basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and environmental damage. (Roger L. Brockenbrough, Kenneth J. Boedecker, 2003).Geometric roadway design can be divided into three parts that are road alignment, road profile, and cross-section. Combined, they provide a three-dimensional layout for a roadway. The alignment is the path that a road follows, as defined by a series of curves and horizontal tangents. The road profile is the vertical aspect of the roadway, it includes sag curves, crest and the straight grade lines joining them. The cross section shows the position and number of vehicle lanes and sidewalks, along with their cross slope or banking. Cross sections also show drainage features, pavement structure and other items outside the category of geometric design.A negative relationship between radius of curve and accident rate was established by (Choueiri et al., 1994).

The same radius of curve in a sequence of similarly tuned radii can have effects on the accident situation other than those in a non tuned sequence of different radii, as is usually the case on most old alignments (Choueiri et al., 1994). Also a significant, positive relationship between sight distance and radius of curve was established. A study was conducted by Feng-Bor Lin (1990) on the flattening of horizontal curves on rural two lane highways and he found out that horizontal curves on highways are on average more dangerous as compared to tangent sections. As the curvatures increase, horizontal curves tend to have an increased accident rate. Thus, curvatures can be used as a primary safety indicator for curves and curve designs.(Y. Hassan et al. 2003) researched the effect of vertical alignment on driver perception of horizontal curves. He found out that the perception of the driver on road features ahead of him or her is a very important human factor and should be considered very much when performing road designs. An erroneous perception of the road can lead to actions that may compromise traffic safety. Poor coordination of horizontal and vertical alignments are considered to be a cause of such wrong perceptions.

From a statistical analysis, he suggested that the horizontal curvature seems consistently sharper on points it overlaps with a crest curve and consistently flatter when it overlaps towards a sag curve.(Zhang Yingxue 2009) made an analysis on the relation between traffic safety and Highway horizontal curve and found out that, super-elevation, curve radius, transition curve, widening, and the sight distance have the most important effect on highway traffic accidents. Another study was conducted by (Ali Aram 2010). He researched effective road safety factors on horizontal curves and found out that, geometric features of the curve, cross section, stopping sight distance, roadside hazards, pavement friction and curve coordination affect the safety performance of horizontal curves. He noticed that the degree of horizontal curvature, super elevation, length of curve, shoulder width and transition length are the important independent effective variables. He furthermore suggested that horizontal curves are associated with higher accident rates compared to straight sections of the same length and traffic type.From the statistical analysis performed throughout the past years, it has been discovered that many variables are very significant to cause road traffic accidents in the highways. Elements of Highway alignment geometric such as, the radius of the curve, the sight distance/visibility and the super elevation play a very big role in causing accidents both in plain and steep terrain highway (Garnaik, no date).

All the studies above indicate that higher accident rates are directly proportional to decreasing horizontal radius, poor visibility, and high superelevation and very steep gradients.Establishing ControlA control point is a reference point from which all the survey work that is done will be referenced from. This is a point of known coordinates and elevation. This point is usually established using a static GPS system for increased accuracy. This point is referenced from an international reference system which enables us to accurately map the work done.(Ghilani and Wolf, 1989). In Zimbabwe, the WGS84 is used in relation with the Modified Clarke 1880 datum.Tachy SurveyThis is a method of determining the horizontal distance, the direction and the elevation of points in relation to the instruments height by sighting with the instrument’s telescope on a graduated rod. The telescope on the tachymeter is sighted on a rod located at the point of interest. The distance, the direction and the height between the elevations of the instrument are used to compute the point’s polar coordinates (Chebotarev, A. S. Geodezija, 2nd ed., part 1. Moscow, 1995.)

Elements of road design:

Road Alignment / Horizontal aspect

The intersection of a road must be provided with certain degree of curve which is determined by its radius, so that when a vehicle is moving towards the corner of a road, the driver of the vehicle will not feel any sudden changes while driving and will have a comfortable driving experience. This curve is known as horizontal alignment and usually provided on a horizontal profile of a road, which is simply known as the plan view. The road curve is provided based on its relation with design speed.Diagram showing a typical Horizontal curve (Garnaik, no date)Where

Δ: Total deflection angle

L: Total length of curveIP: Intersection point of tangents

ST: Short tangentRC: Radius of circular curve

LT: Long tangentLS: Length of spiral curve

Ts: Total tangent distance

LC: Length of circular curve

Es: External distance

The initial design is usually based on a series of straight sections whose positions are defined by the topography of the area. When two straights intersect, they are joined by a horizontal curve. Main types of road curves are circular curves of constant radius, transition curves of varying radius (spiral) and vertical curves of parabolic form (Schofield and Breach, 2007).Curve calculations are based on the arc definition for a circular curve. Sufficient curve length must be used in open country to prevent the appearance of a "kink" in the line. At points of small deflection angles, a minimum arc length of 15 times the design speed is required. At points of larger deflection angles where spiral transitions are necessary, the minimum arc length of the curve is 15.24 meter. An angle point is considered a curve with an arc length of zero, and therefore, does not meet the minimum standard of design (Manual, 2012.). Compound curves are adjoining curves in the same direction with differing degrees of curvature. They may be used where necessary with an intermediate spiral segment. Broken back curves are curves in the same direction connected with a short segment of tangent. It is desirable to avoid the use of broken back curves. When the use of a broken back alignment cannot be avoided, design the tangent section so that all travel lanes slope in the same direction as the super elevation of the curves. This avoids the introduction of two flat spots on the travel lane toward the outside of the curves and prevents the development of a dip on the edge of the pavement that can affect driver comfort and drainage (Manual, 2012.)Road Profile/ Vertical aspect

Road Cross section

Cross section is defined as the number of lanes and lane width including cross fall, shoulder, sidewalk, earth slope and drainage features in the transverse direction of the roadway. The cross section shows the total formation of a roadway (Garnaik, no date).

Sight distance

The sight distance on a roadway is a safety design factor, which is directly linked to rate of change of grade. Drivers approaching from both sides of a particular vertical curve cannot see each other not until they arrive on the crest almost simultaneously of which it might be too late to prevent the occurrence of an accident. If the curve had been longer and flatter thus resulting in a low rate of change of gradient, the drivers would have had a longer sight distance and could have had more time in which to take avoiding actions. Thus, sight distance, that is the length of road ahead that is visible to the driver, is a safety factor, and it is obvious that the sight distance must be greater than the stopping distance in which the vehicle can be brought to rest (Schofield and Breach, 2007).

Factors that influence horizontal road curvature designs near roundabouts and roundabout design.

Standards for sight distance, pavement width, shoulder length and superelevation of highway points toward a roundabout.

History of roundabouts

Modern roundabouts are mistakenly confused with old-style traffic circles and rotaries. Traffic circles have been around since long ago centuries, the earliest being said to be constructed in 1905 on the southwest corner of Central Park in New York City. From the beginning, roundabouts have provided the ability for a city or town to join a number of intersecting roads. A number of large roundabouts were constructed in the USA up until the 1950s when they started to fall out of favour. The old-style traffic circles enabled high speed merging and weaving of vehicles that led to a high crash experience (Federal Highway Administration, 2009).

Modern Roundabout

A modern roundabout is that with traffic control features and specific designs that distinguish it from other old type circular intersections. The features include a clockwise traffic flow direction around a central island, yield control for entering traffic, and appropriate geometric curvature and features to induce desirable vehicular speeds. These features have been proven to have the greater ability of reducing the number and severity of intersection accidents (Federal Highway Administration, 2009).

Characteristics of roundabouts

Chapter 3. Research MethodsResearch design

This study is based on a qualitative and quantitative analysis. The qualitative method is used to collect primary data through interviews and questionnaires. Interviews helped the researcher to investigate issues in an in-depth way. Furthermore questionnaires are a faster way of collecting data. The quantitative approach used in the review of records of all traffic accidents which occurred in the study are from the beginning of 2017 up to date.

The purpose of reviewing the records is to assess the patterns and trends of road traffic accidents in the area and to show their relationship with road design elements. A detailed topographic survey on the road section is also produced by the use of data collected on the study area. This is done in order to obtain a topo map and a digital terrain model which is then used to determine conformity of roundabouts to the standards of the Ministry of Transport.

Target population

Officials from various organizations like the Ministry of Transport, Gweru Central Police Station, Traffic Safety Council of Zimbabwe, Fire Brigade and Gweru City Council are targets in this research.

Methods of data collection

Interviews

Interviews are done to specific targeted sources the chief police officer in particular of the Gweru central police. To assess the relationship between geometric design of traffic circles and road traffic accidents especially some detail about the accidents site and the probable geometric design cause. The chief fire officer at Fire Brigade is also interviewed to get information on the fatality of the accidents compared to its location. The chief fire officer keeps records on all road traffic accidents and fires.

The assistant director (engineering services department) is also interviewed to help discuss the probable solutions to the accident problem in the study area. The assistant director (engineering services department) is responsible for town planning and designation of suitable areas where horizontal curves should be constructed and to ensure they conform to the Ministry of Transport standards.

ProcedureInstrument Total StationPoints Collection. An arbitrary point is selected close to the study area. Side shorts are taken across the road section including the roundabout. This data will be used to create a tachy map that is later used to redesign the road layout. This design will show the geometric parameters of the section and weather they are of the desired standards of the Ministry of transport.The following segment characteristics are identified for the horizontal curve and for the associated tangent section: Outside shoulder width Inside shoulder width Median width (without inside shoulders)Sight distancePavement widthThe following characteristics are identified for the roundabout design:Kerb radius Entry angle Inscribed circle diameterData analysis and presentation of resultsData range of curve geometry characteristics

Minimum

Average Maximum

Radius

Total Length of curve

Lane width

Shoulder width

During the field work data on position of accident and accident fatality are displayed and a comparison is made against the geometric parameters of the road section. Digital CAD diagrams showing sight distance, pavement width, shoulder length and superelevation are created using Surpac and geometric analysis is done using AutoCAD. A comparison is then made between the existing geometric parameters and the Ministry of Transport standards to check if the road cross section conform to the design standards. Multiple linear regressionThe multiple linear regression model, which expresses the relationship between the dependent variables and the independent variable as a straight-line is fitted to the data. The model allows for all possible variable combinations to be examined.