road accidents and the related mass casualties are one of the challenges of the human society that threaten the health of people and impose huge costs on the states . developed countries have the largest number of victims due to road accident and is considered as one of the main causes of death . the degree of injuries and the frequency of major road accidents are to such extent that it is called the according to the statistics , on average , 10% of the injuries lead to death and about 3,000 people are killed per day due to traffic accidents around the world . similar to other human factors , the behavior of a driver is influenced by conscious and unconscious factors called  cognitive - behavioral characteristics  . based on a study , driving errors ( i.e. human factors ) are responsible for 93% of traffic accidents , 34% due to operational deficiencies , and 12% due to vehicle malfunctions . the results confirm reports that indicate the role of human factors ( individual or social ) in road accidents . moreover , other studies have highlighted the role of driver s personality and visibility in driving behavior . according to past research studies , drivers that cause accident are distinguished from other drivers by factors such as empathy , characteristics , stress level , and few other parameters . an analysis of the road accidents in iran also revealed several factors for such occurrences . showed that the contribution of human factors to road accidents is 97.5% and the role of human factors in causing an accident is 49% . in their research , hassanpour et al . found that human factors are one of the most important causes of driving accidents in iran ( more than 30% of accidents and 23% of driving losses ) . moreover , refahi et al . stated that the role of attitude in driving behavior is more important than other aspects . consistent with their opinion , there was an indirect association between variables such as personality traits and driving behavior . in general , human factors that are involved in road accidents are categorized into two general classifications , namely cognitive - behavioral factors and behavioral factors . for the latter , behavioral  corresponds to the contributing factors ( e.g. infringement of driving laws , driving at high speed while feeling dizzy and fatigued , drug or alcohol use ) in causing a road accident . while recognizing the adverse effect of road conditions and vehicle health in causing accidents , researchers have hypothesized that the effect of human factors is omitted and should be investigated . the discarded factors are personality traits ( based on the big five personality traits ) , driving behavior , and mental disorders ( based on the diagnostic interview ) . this study was conducted to evaluate the specific hypothesis that the cognitive - behavioral factors and mental disorders can predict road accidents . the aim of this cohort study was to identify the psychological factors that influence road accidents . in addition , we compared driving behavior and its subscales in two groups of drivers with accidents and those without road crash . due to increased fatalities and an upsurge in road trips in iran , our country is also not immune from road accidents . in this cohort study , 800 bus and truck drivers were recruited among those referred to imam sajjad hospital ( tehran , iran ) during october 2013 to may 2015 . these drivers referred to the hospital for a drug test , as a medical pre - requisite for renewing their driver s license . as a profession , they were heavy vehicle drivers and were holding category c or d driver s license . we intentionally targeted heavy vehicle drivers since they are in high - risk accident category compared to other drivers . convenience sampling , a non - probability sampling technique , was used in this study . initially , under the supervision of interviewers , the participants were encouraged to fill in the questionnaire forms . this was followed by a semi - structured interview ( schizophrenia and affective disorders scale [ sads ] ) by a clinical psychologist to diagnose any mental disorders . the questionnaires comprised of demographic questions , manchester driving behavior questionnaire ( mdbq ) , neo personality inventory , and smoking and substance abuse inventory . the inclusion criteria were male drivers , holders of category c or d driver s license , age range 23 - 85 years and being referred to imam sajjad hospital . the exclusion criteria were female drivers , suffering from severe mental disorders ( e.g. chronic psychosis or dementia ) , and illiteracy or uneducated drivers unable to understand the questions . then , based on police accident reports , we identified those drivers that were involved in a driving accident due to human factors . the demographic questions consist of participants age , education , history of driving , the amount of kilometers traveled per month , marital status , time taken to obtain driver s license , and the number of occurred accidents caused by human factors . this scale was adjusted and compiled by rissen et al . in the psychology department of manchester university , manchester , it is based on the idea that errors and violations have different psychological reasons and correction methods ; hence , they should be discriminated by researchers . the questions cover two different aspects ; the kind of behavior and the amount of risk posed to other drivers . these behaviors are classified as follows : 
 behaviors that pose no risk to others and merely give a feel of comfort ( low - risk probability)behaviors that are likely to put others at risk ( moderate - risk probability)behaviors that certainly put others at risk ( high - risk probability ) . behaviors that pose no risk to others and merely give a feel of comfort ( low - risk probability ) behaviors that are likely to put others at risk ( moderate - risk probability ) behaviors that certainly put others at risk ( high - risk probability ) . obtained a correlation coefficient of 0.81 for errors and 0.75 for violations in another reliability research conducted on 80 drivers with a 7-week interval . iliescu and srbescu reported that the reliability of mdbq factors is from 0.62 to 0.78 , which is satisfactory . moreover , oreizi reported that the iranian version of mdbq has acceptable reliability and validity . in their study , the reliability of the factors ranged from 0.65 to 0.81 . paul costa ( 1985 ) designed this questionnaire as a short form of the neo personality inventory . each question inquired about one of the main five personality dimensions , including neuroticism ( n ) , extraversion ( e ) , openness to experience ( o ) , agreeableness ( a ) , and conscientiousness ( c ) , respectively . each of the factors covered 12 questions and each scale scored from 0 to 48 . the validity and reliability of the persian version of neo personality inventory have been confirmed in various studies . yadollahi et al . noted that the short form of neo has acceptable reliability ( cronbach  between 0.69 - 0.83 ) . this questionnaire consists of 12 items , with five - point likert spectrum , and it surveys the dosage of the drugs , stimulants , hallucinogens , and alcohol during the past year . clinical psychologists conducted the semi - structured interview to diagnose any mental disorder in drivers . this interview evaluated 16 main psychology and psychiatric disorders and other psychiatric disorders , including psychiatric problems , depression , mania , hypomania , substance abuse and addiction , psychosis , anxiety disorders , antisocial personality , somatization disorders , suicidal thoughts , post - traumatic stress disorders ( ptsd ) , dissociative disorders , epilepsy disorder , alzheimer , and mental retardation . simpson et al . reported that cohen s kappa coefficient of mania , hypomania and depression was calculated as 0.83 , 0.72 and 1 , respectively . the diagnostic interviews were performed by clinical psychologists that had been educated in the field of diagnosis and treatment of mental disorders . data were analyzed using the statistical package for social sciences ( spss ) version 18.0 . in addition , descriptive statistics were used to demonstrate demographic data . to compare driving behavior and its subscales the effective factors that contribute to road crashes were determined using multiple logistic regression analysis . p values less than 0.05 were considered as statistically significant . to analyze the data , all variables including personality traits , mental disorders , and demographic characteristics that were collected through questionnaires and were likely to have an impact on road accidents were identified . the logistic regression analysis was conducted for each variable in the univariable analysis and crude or was also estimated . in the next step , variables with p values then , those variables that had a significant role in the model with p values<0.05 were interpreted with adjusted or . in this cohort study , 800 bus and truck drivers were recruited among those referred to imam sajjad hospital ( tehran , iran ) during october 2013 to may 2015 . these drivers referred to the hospital for a drug test , as a medical pre - requisite for renewing their driver s license . as a profession , they were heavy vehicle drivers and were holding category c or d driver s license . we intentionally targeted heavy vehicle drivers since they are in high - risk accident category compared to other drivers . convenience sampling , a non - probability sampling technique , was used in this study . initially , under the supervision of interviewers , the participants were encouraged to fill in the questionnaire forms . this was followed by a semi - structured interview ( schizophrenia and affective disorders scale [ sads ] ) by a clinical psychologist to diagnose any mental disorders . the questionnaires comprised of demographic questions , manchester driving behavior questionnaire ( mdbq ) , neo personality inventory , and smoking and substance abuse inventory . the inclusion criteria were male drivers , holders of category c or d driver s license , age range 23 - 85 years and being referred to imam sajjad hospital . the exclusion criteria were female drivers , suffering from severe mental disorders ( e.g. chronic psychosis or dementia ) , and illiteracy or uneducated drivers unable to understand the questions . then , based on police accident reports , we identified those drivers that were involved in a driving accident due to human factors . the demographic questions consist of participants age , education , history of driving , the amount of kilometers traveled per month , marital status , time taken to obtain driver s license , and the number of occurred accidents caused by human factors . this scale was adjusted and compiled by rissen et al . in the psychology department of manchester university , manchester , it is based on the idea that errors and violations have different psychological reasons and correction methods ; hence , they should be discriminated by researchers . the questions cover two different aspects ; the kind of behavior and the amount of risk posed to other drivers . these behaviors are classified as follows : 
 behaviors that pose no risk to others and merely give a feel of comfort ( low - risk probability)behaviors that are likely to put others at risk ( moderate - risk probability)behaviors that certainly put others at risk ( high - risk probability ) . behaviors that pose no risk to others and merely give a feel of comfort ( low - risk probability ) behaviors that are likely to put others at risk ( moderate - risk probability ) behaviors that certainly put others at risk ( high - risk probability ) . obtained a correlation coefficient of 0.81 for errors and 0.75 for violations in another reliability research conducted on 80 drivers with a 7-week interval . iliescu and srbescu reported that the reliability of mdbq factors is from 0.62 to 0.78 , which is satisfactory . moreover , oreizi reported that the iranian version of mdbq has acceptable reliability and validity . in their study , the reliability of the factors ranged from 0.65 to 0.81 . paul costa ( 1985 ) designed this questionnaire as a short form of the neo personality inventory . each question inquired about one of the main five personality dimensions , including neuroticism ( n ) , extraversion ( e ) , openness to experience ( o ) , agreeableness ( a ) , and conscientiousness ( c ) , respectively . each of the factors covered 12 questions and each scale scored from 0 to 48 . the validity and reliability of the persian version of neo personality inventory have been confirmed in various studies . noted that the short form of neo has acceptable reliability ( cronbach  between 0.69 - 0.83 ) . this questionnaire consists of 12 items , with five - point likert spectrum , and it surveys the dosage of the drugs , stimulants , hallucinogens , and alcohol during the past year . clinical psychologists conducted the semi - structured interview to diagnose any mental disorder in drivers . this interview evaluated 16 main psychology and psychiatric disorders and other psychiatric disorders , including psychiatric problems , depression , mania , hypomania , substance abuse and addiction , psychosis , anxiety disorders , antisocial personality , somatization disorders , suicidal thoughts , post - traumatic stress disorders ( ptsd ) , dissociative disorders , epilepsy disorder , alzheimer , and mental retardation . simpson et al . reported that cohen s kappa coefficient of mania , hypomania and depression was calculated as 0.83 , 0.72 and 1 , respectively . the diagnostic interviews were performed by clinical psychologists that had been educated in the field of diagnosis and treatment of mental disorders . data were analyzed using the statistical package for social sciences ( spss ) version 18.0 . in addition , descriptive statistics were used to demonstrate demographic data . to compare driving behavior and its subscales the effective factors that contribute to road crashes were determined using multiple logistic regression analysis . p values less than 0.05 were considered as statistically significant . to analyze the data , all variables including personality traits , mental disorders , and demographic characteristics that were collected through questionnaires and were likely to have an impact on road accidents were identified . the logistic regression analysis was conducted for each variable in the univariable analysis and crude or was also estimated . in the next step , variables with p values < 0.2 were selected and entered into the multiple logistic regression analysis . then , those variables that had a significant role in the model with p values<0.05 were interpreted with adjusted or . their age ranged from 24 to 81 years with an average age of 46.711.17 years ( meansd ) . the majority of the 654 drivers ( 81.7% ) were married , 730 ( 91.2% ) had a secondary school education or less , and 70 ( 8.8% ) were college graduates . summary of demographic information from 800 bus and truck examinees driving history : number of years of driving per person table 2 shows the mean , standard deviation , and comparison of the driving behavior and its subscales between the two groups of drivers with road accidents and without road crash . the t test analysis indicated that the two groups differed significantly in terms of driving behavior and all its dimensions such as slips , deliberate violation , laps error , and unintentional violation ( p<0.05 ) . the mean , standard deviation , and comparison of driving behavior and its subscales between the two groups of drivers with accidents and without road crash driving behavior : scores of driver s driving behavior the findings of this study revealed that age reduced the risk of accident occurrence ( positive effect ) , indicating that by per year increase in age , the odds of accident occurrence was reduced by 0.01-fold . in addition , we found that driving history also reduced the risk of accidents ( positive effect ) , meaning that by per year increase in driving history , the odds of accident occurrence were reduced by 0.05-fold . based on the multiple logistic regression interpretations ( multivariable analysis ) , accident history had no impact on the risk of accident occurrence ( table 3 ) . multiple logistic regression - independent predictors of road accident in truck and bus drivers ( n = 800 ) the results revealed that depression , in three levels of minor , mild and major , increased the odds of road accidents by 2.4-fold . furthermore , with regard to anxiety disorders , particularly obsession disorder , the results revealed that this disorder increased the odds of road accidents by 2.7-fold . ptsd and driving behavior were not significant parameters in the equation ; and among the personality traits , neuroticism was more likely to increase crash probability by 1.1-fold , but other personality factors did not seem to have a significant effect on the risk of accident occurrence . moreover , since none of the drivers mentioned any disorders , such as smell or vision delusion , psychosis symptoms , epilepsy , mania , hypomania , panic disorder , phobia , fugue , or other mental problems , we therefore did not enter any of these disorders into the equation . the aim of this cohort study was to identify the possible impact of personality traits , some mental illnesses , and driving behavior of bus and truck drivers on the risk of road accidents . after two years of follow - up , we surveyed those samples that had road accidents due to human factors . based on the results of this study , older age reduces the occurrence of accidents ( positive effect ) , which means that per year of age the odds of accident occurrence is reduced by 0.01-fold . for example , in a substantial research done by heidari et al . , they reported that there was a significant relationship between age and all the errors and infringements of driving . the risk of driving violations is higher in youths aged 18 - 25 years old , as this age range is one of important risk factors for road accidents . on the other hand , mehmandar et al . argued that older age and other demographic properties such as a university degree and female gender seem to be the most vital risk factors in road accidents that lead to fatalities in iran . clearly , driving is a complex activity and includes cognitive , visual , and bodily processes . hence , driving could be adversely affected by factors such as age and mental illness that can affect cognitive processes involved with driving . therefore , it can be stated that older people are more prudent and less likely to be involved in a road accident . moreover , the ability to drive safely and skillfully is a process that takes 8 to 9 years . the results of our study suggest that it will take several years for a novice driver to turn into an experienced driver . to clarify the relationship between age and reduction in the occurrence of traffic accidents , we can state that older people are more experienced and cautious than young drivers . awareness of mental and physical limitations is more realistic in older drivers , so they exhibit self - control behavior that leads to a better risk estimation while driving . the results revealed that driving history could reduce the likelihood of accidents ( positive effect ) , meaning that as driving experience is enhanced every year and as a driver reaches an older age , the chance of an accident will reduce . in other research studies , it is stated that compared with a history of car accidents , the duration of a driver s license is more associated with the reduced risk of accidents due to human factors ; and more experience is associated with reduced accidents . based on the results of a study , having visited a psychiatrist or psychologist did not affect the risk of accident occurrence . this result is in contrast with those of previous research studies ; for instance , a study noted that tired , mindless , careless and anxious drivers experience difficulty while driving and they sometimes exhibit risky driving behavior . in another study , it was mentioned that patients with psychiatric disorders had twice as many accidents compared with a control group . the inconsistency between our results and those of the previous studies could be due to the utilization of different sample groups and diagnostic tools . in iran , referring to a psychologist or psychiatrist is occasionally considered taboo , which in turn could negatively affect the results . the preliminary results revealed that depression in three levels of minor , major and mild was a predictor of an accident and increased the odds of accidents by as much as 2.4-fold . in a similar research , it was found that risky driving behavior is associated with sensation seeking , lack of tolerance , stress intolerance , impulsivity , violence , emotional instability , adventure , depression and pessimistic thoughts . another research found a positive and robust correlation between depression and risky driving behavior , especially in injury accident and driving while intoxicated . da silvia - junior et al . noted that truck drivers were more susceptible to depression in comparison with the general population and depression prevalence among them was 13.6% . other studies mentioned that patients who suffer from psychiatric illnesses ( such as depression or anxiety ) have a double accident versus a control group . the results revealed that mania and hypomania , as well as a history of psychiatric problems , did not increase or decrease the risk of accidents . therefore , it can be concluded that these results arise from the lack of proper sample size or the absence of mania or hypomania symptoms in the sample group . in anxiety disorders spectrum , especially obsession disorder , the results demonstrated that obsession disorders increased the odds of accidents by 2.7-fold in truck and bus drivers . previous reports demonstrated that a significant correlation was found between neurosis such as anxiety and depression with road crashes . however , in other research , significant differences were reported between the scores of paranoid thoughts , obsessive and compulsive behavior , and interpersonal sensitivity in both groups of drivers with and without road accidents . asghari et al . reported that there was a negative correlation between anxiety and driving psychology . overall , it can be assumed that mental health is one of the variables that has been investigated in aggressive behaviors of drivers . it is believed that persons , who suffer from mental disorders , may jeopardize the health of themselves and others with their driving behavior . based on our findings , among the personality traits , only neuroticism increased the risk of accidents and other personality traits were not significant in the logistic regression equation . these findings are consistent with studies by wang , rau , and solvency . they emphasized the role of vision and personality factors in road accidents . another research also showed a positive correlation between the neuroticism index with a variety of errors and illegal acts in driving behavior , which confirms the results of this study . moreover , a large study , which was conducted in 34 countries indicated that there was a negative correlation between neuroticism and traffic fatalities . in another study , no positive relationship was found between aggressive driving and extroversion or sensation seeking . on the other hand , fyhri and is also negatively associated with risky driving or is correlated with aggressive driving , which is inconsistent with our results . the inconsistency between our results and those in the above - mentioned studies could be due to different populations , personality types , and the effect of other variables such as age , environmental factors , and history of driving . in one of the studies , the participants were young drivers whereas in our study the drivers were selected from all age ranges , which could have affected our results . it should be noted that the complexity of human behavior stems from various factors such as cultural , educational , economic , social factors and mental health , which can affect the driving behavior of truck and bus drivers and may play an essential role in shaping their behavior . first , data were collected over a very short period of time ( 2 years ) and administering the questionnaires ( mdbq and neo ) had certain constraints . second , the procedure for selecting the samples did not allow us to generalize the results to other populations such as those holding category a and b driver s licenses . the two main factors in the incidence of traffic accidents are the human factors and patterns of driving behavior . in this study , it was observed that the underlying factors of traffic accidents were human , cognitive , and behavioral factors such as mental illness , personality , or the age of drivers . using the results of this study , we can control the percentage of road accidents in drivers holding category d and c driver s licenses . in addition , by introducing necessary changes in the process of obtaining a driver s license ( e.g. performing a psychological test ) we can guide drivers to shape a safe driving behavior . the strengths of this study were its cohort study design and diagnostic interview of all participants to diagnose mental disorders . since it has yet to be established whether the quality of road and vehicles are the cause of road crashes , researchers need to conduct longitudinal research on road accidents and perform the psychological tests precisely .