Does the Type of Cross Section and Type of Intersection Affect Safety on Bypasses?

Abstract

This paper presents a comprehensive analysis of the impact of the road cross-section configuration and junction types on traffic safety along selected bypasses in Poland. The study evaluates the safety implications of different design alternatives based on empirical data collected from 18 bypasses, comprising both single and dual carriageway sections. The findings indicate that, although single-carriageway bypasses exhibit a lower absolute number of accidents and collisions, they demonstrate a higher concentration of such accidents or collisions compared to dual-lane bypasses. Furthermore, the severity of accidents is significantly higher on single-carriageway bypasses. The analysis also reveals that the majority of collisions, irrespective of carriageway type, occur at at-grade junctions and roundabouts, with only 20% occurring at grade-separated interchanges. This suggests that the implementation of grade-separated interchanges contributes to enhanced road safety outcomes. Additionally, the study concludes that, within the context of ring roads, roundabouts offer a safer alternative to traditional at-grade junctions.

1. Introduction

The increasing presence of both trucks and passenger vehicles on urban road networks in Poland has led to escalating congestion, particularly within city centers. This surge in traffic volumes has been directly associated with a rise in road traffic accidents, reduced traffic flow efficiency, and a decline in the overall quality of life for urban residents. Notably, this phenomenon is not confined to major metropolitan areas but also affects smaller towns across the country. One of the principal strategies employed to mitigate these challenges is the construction of bypass roads. In recent years, the Polish government has initiated several infrastructure development programs aimed at expanding the national road network and improving traffic safety. Among these, the Program for the Construction of 100 Bypasses for 2020–2030 is of particular significance. Bypasses serve a critical function by redirecting transit traffic away from city centers, thereby enhancing road safety, improving traffic flow, and contributing to improved living conditions for local communities.

While human behavior remains the predominant factor influencing road safety, the technical characteristics of the road infrastructure also play a vital role in ensuring safe traffic operations. Roads that are well-designed, clearly marked, and properly maintained positively influence driver behavior and significantly reduce the likelihood of hazardous situations. Consequently, the enhancement of traffic conditions and road safety is a fundamental responsibility of road authorities and traffic engineers. However, financial constraints often compel road administrators to adopt more cost-effective solutions, which may compromise safety outcomes. Given the increasing number of road accidents and collisions and the rapid expansion of Poland’s road network, it is essential to investigate the influence of specific technical parameters—such as carriageway width, number of roadways, lane configuration, and intersection design—on traffic safety performance.

This study aims to analyze the impact of road cross-section design and intersection types on traffic safety across selected Polish bypasses. Based on empirical research conducted on 18 bypasses—comprising single and dual carriageway configurations—the paper evaluates the risks and benefits associated with different design solutions. The objective is to identify effective infrastructure strategies that can improve traffic safety on ring roads.

2. Literature Review

Research on the social implications of bypass construction has been carried out in several European countries, particularly in the UK, Germany, and the Netherlands. These studies were primarily undertaken during the period of rapid road network expansion between 1970 and 1995. In contemporary infrastructure planning, it is now standard practice to incorporate social impact assessments, including public opinion surveys, into the decision-making process for bypass projects. A critical component of these assessments involves evaluating the perceived benefits and drawbacks of bypass construction among the most affected social groups. Common concerns include land expropriation, environmental degradation (e.g., air and noise pollution), and disruption to local traffic patterns. To ensure the reliability of findings, the survey should be conducted both prior to and following the completion of the bypass. Post-construction surveys are particularly valuable, as they provide empirical insights into the actual impact of infrastructure on local communities. Public awareness of the full consequences of such investments often emerges only after the infrastructure becomes operational [1]. In this study, surveys conducted before and after the opening of the Girarduf bypass revealed significant discrepancies in public perception. Pre-construction concerns, such as increased noise, property devaluation, and environmental risks, were largely unsubstantiated in post-construction evaluations. Two distinct groups were surveyed: residents living along the former transit route and those residing near the newly constructed bypass. The former group reported a marked improvement in the quality of life, citing reductions in traffic volume, especially heavy vehicles, and a noticeable decline in noise levels. The latter group initially expressed apprehensions regarding land fragmentation, increased noise, drainage issues, and structural vibrations. However, post-construction feedback showed that most residents’ concerns were premature, with the primary negative outcome being a decline in property values, making it difficult to sell. Public opinion on bypass construction remains divided. While some residents view such projects as opportunities for economic development, others express concern over potential declines in local commerce, particularly in small towns where bypasses may divert customer traffic away from central business districts. This dichotomy is supported by international studies. For instance, research conducted in Indiana, USA, demonstrated that bypass construction in large cities contributed to increased employment and wage growth, particularly in the manufacturing sector [2]. However, the economic impact of bypasses is closely tied to the urban scale. Larger cities tend to absorb the effects more effectively, whereas smaller towns may experience adverse outcomes, such as reduced retail activity in town centers.

The spatial redistribution of commercial activity is another notable consequence. New businesses often emerge along bypass corridors, shifting economic activity from traditional urban cores to peripheral zones [3]. While this may elevate income levels in outlying areas and stimulate logistics and industrial development, it can also undermine the economic vitality of central districts. Moreover, bypasses frequently traverse previously undeveloped land, creating opportunities for new investments and job creation. However, inadequate consideration of local economic structures—particularly in tourism-dependent areas—can result in unintended negative consequences, such as reduced visitor appeal due to increased noise and pollution.

Therefore, comprehensive socio-economic and demographic analyses are essential during the planning phase. These should be complemented by robust social impact assessments to ensure that infrastructure development aligns with local needs and minimizes adverse effects [4].

In addition to social and economic considerations, traffic safety remains a central objective of bypass construction. While projected safety impacts are typically assessed during the design phase, post-construction evaluations are less common. One notable exception is the study “Analysis of road traffic safety improvement on the example of two bypasses in Mazovia” [5], which examined the Wyszków bypass and the Garwolin bypass. The findings indicated a substantial reduction in traffic-related injuries on the DK8/S8 and DK17/S17 national roads—from over 40 injuries in 2006 to fewer than 10 in 2012—following the implementation of the bypasses. However, no significant safety improvements were observed on adjacent municipal roads.

A similar study conducted in New Jersey, USA, assessed the impact of three bypasses on suburban and urban traffic patterns. While the bypasses successfully diverted transit traffic from city centers and improved regional accessibility, high traffic volumes persisted in urban cores due to route preferences among drivers. Nonetheless, the bypasses achieved their primary objectives of enhancing the safety and traffic flow within the towns [6]. The selection of the bypass cross section is based on an analysis of the traffic volumes and the functions to be performed in a given area. In Poland, both single-lane, 2 + 1 cross-section bypasses, and dual-lane bypasses are designed. In areas with lower traffic volumes, a single-lane cross section may be sufficient. whereas dual-lane configurations are preferred in high-traffic areas to enhance capacity and reduce congestion. Due to financial constraints, the 2 + 1 cross section has emerged as a cost-effective alternative, offering overtaking opportunities and traffic flow. However, both in Poland and abroad, there is a lack of specific design guidelines specific to 2 + 1 bypasses. Existing regulations tend to address bypasses and 2 + 1 road sections separately, highlighting the need for integrated design standards [7]. Recent studies [7] have examined the bypass of Girarduf, a 15 km long road segment constructed with a 2 + 1 cross section, characterized by alternating overtaking lanes. This bypass was selected for analysis due to its high traffic volume and a notably elevated proportion of heavy vehicles, with trucks accounting for approximately 40% of total traffic. The findings highlight several operational and safety challenges associated with the 2 + 1 configuration, particularly under conditions of intense freight traffic.

One of the primary issues identified is the obstruction caused by heavy vehicles traveling in platoons, which undermines the intended function of the 2 + 1 layout—namely, facilitating safe overtaking maneuvers. International research suggests that under such conditions, the effective capacity of 2 + 1 roads may be lower than that of conventional single carriageways, with the excessive share of trucks being a critical limiting factor [7]. Safety analyses of the Girarduf bypass revealed a concentration of traffic incidents near the termination points of overtaking lanes. These incidents were frequently attributed to misjudgments by drivers regarding the available overtaking distance, resulting in abrupt lane changes and unsafe merging behavior. Although Girarduf represents an outlier in terms of accident frequency among Polish 2 + 1 bypasses, the case underscores the potential risks inherent to this design under specific traffic conditions [8]. Broader studies on overtaking lane implementation [9] have demonstrated a substantial reduction in traffic accidents—47% overall and 49% for multi-vehicle collisions—following the introduction of such lanes. However, these benefits were partially offset by increased accident rates at entry and exit segments. Traffic efficiency gains were modest, with a 6.6% reduction in vehicle platoon density and a 10.5 km/h increase in average speed. The effectiveness of overtaking lanes was found to correlate with their length, with longer segments enabling safer and more frequent overtaking. Subsequent research [10] emphasized the heightened risk in areas with a heavy vehicle share exceeding 50%, which impairs visibility and traffic fluidity. Key determinants of conflict frequency included the overtaking lane length, traffic volume, and time-to-collision (TTC). Short overtaking segments (<1000 m) significantly increased the likelihood of unsafe maneuvers due to limited execution time, particularly during peak traffic periods. A comparative study conducted in Florida [11] assessed the conversion of single carriageways to dual carriageways with central dividers. The results indicated a marked reduction in accident rates—over 63% in urban areas and 45% in rural zones—suggesting that the safety benefits of such upgrades are more pronounced in high-density environments. The additional lane capacity facilitates smoother overtaking and alleviates congestion, contributing to improved traffic safety. Intersection design on bypasses also plays a pivotal role in traffic performance and safety. The selection of intersection type is influenced by factors such as road classification, traffic volume, design speed, and land availability [12]. While grade-separated interchanges offer superior traffic flow, they require significantly more space. Accidents at intersections are often linked to inadequate signage, poor visibility, and insufficient merging lengths [13]. Critical safety zones include entry/exit ramps and weaving sections, where abrupt lane changes are common. Modern interchange designs—such as diverging diamond interchanges (DDIs) and roundabouts—have demonstrated lower rates of severe accidents compared to traditional diamond or cloverleaf configurations. The complex geometry of cloverleaf interchanges increases conflict points, whereas simplified designs like single-point urban interchanges (SPUIs) reduce them. High traffic volumes exacerbate accident risks, highlighting the importance of incorporating projected traffic data into design decisions [14].

Furthermore, improper design or maintenance of entrance and exit ramps, as well as weaving sections, significantly elevate accident risk [15]. In particular, unsafe lane changes by heavy vehicles at interchanges are a major concern. Research indicates that ramp geometry—including the deceleration lane length, ramp type, and curvature radius—directly affects the feasibility and safety of mandatory lane changes by trucks [16].

Studies on intersection type selection aimed at maximizing safety suggest that optimizing the individual geometric elements of intersections is more effective than applying standardized models without thorough analysis [17]. Concurrently, research has shown that increasing the number of lanes in a road cross section correlates with greater incident severity [18].

The reviewed literature indicates that the topic of bypass design is not widely studied. While considerable attention has been given to diverting transit traffic from urban centers to bypasses, this shift can lead to complications on inadequately prepared access roads. Existing studies tend to focus on the detailed geometric design of specific junctions or interchange segments. However, there is a noticeable gap in the macroscopic analysis of bypass construction, particularly regarding the selection of cross-sectional profiles and intersection type. This paper aims to address this gap by examining these broader design considerations.

3. Materials and Methods

Eighteen bypasses from different parts of Poland were surveyed, including nine with a single-lane cross section and nine with a dual-lane cross section. All of them were put into service from 2014 to 2019. Data were collected on their technical parameters such as the road technical class, type of cross section, length, design speed, lane width, median width, shoulder width, and the type and number of crossroads. The data are presented in two tables: for single-carriageway (

Table 1. Summary of the parameters of the analyzed single-carriageway bypasses.

No.	City Bypass	Road Technical Class	Cross Section	Length [km]	Design Speed [km/h]	Lane Width [m]	Median Width [m]	Shoulder Width [m]	Number of Interchanges	Number of Roundabouts	Number of Unsignalized Intersections
1	Augustów	GP	1x2	20.00	100	3.50	-	2.00	4	-	-
2	Bargłów Kościelny	GP	1x2	11.80	100	3.50	-	1.50	-	-	3
3	Bełchatów	GP	1x2, 2 + 1	10.90	80	3.50	-	1.50	4	-	-
4	Hrubieszów	GP	1x2	9.27	80	3.50	-	1.50	-	2	3
5	Kłodzko	GP	1x2	6.60	80	3.50	-	1.50	1	1	-
6	Kościerzyna	GP	1x2, 2 + 1	7.60	80	3.50	4.00	1.50	4	-	-
7	Leżajsk	GP	1x2	8.00	80	3.50	-	2.00	2	1	2
8	Skawina	GP	1x2	5.30	60	3.50	-	2.75	-	4	2
9	Wieluń	GP	1x2, 2 + 1	13.24	100	3.50	-	1.50	3	-	-

) and dual-carriageway (

Table 2. Summary of the parameters of the analyzed dual carriageway bypasses.

No.	City Bypass	Road Technical Class	Cross Section	Length [km]	Design Speed [km/h]	Lane Width [m]	Median Width [m]	Shoulder Width [m]	Number of Interchanges	Number of Roundabouts	Number of Unsignalized Intersections
10	Gorzów Wielkopolski	S	2x2	11.66	100	3.50	2.50	1.25	3	-	-
11	Góra Kalwaria	GP	2x2	8.96	80	3.50	2.00	0.75–2.25	3	-	-
12	Inowrocław	GP	2x2	25.10	80	3.50	2.50	1.50	5	-	-
13	Nysa	GP	2x2, 2 + 1	16.50	80	3.50	-	1.50	1	4	-
14	Ostróda	GP	2x2	8.50	100	3.50	2.50	1.25	1	1	-
15	Ostrów Wielkopolski	S	2x2	12.80	100	3.50	2.50	1.25	1	1	-
16	Radom	S	2x2	24.65	100	3.50	2.50	0.75–4.00	4	-	-
17	Suwałki	S	2x2	12.83	100	3.50	2.50	0.75–2.80	3	-	-
18	Szczecinek	S	2x2	11.98	100	3.50	2.50	0.75	2	2	-

) roads separately.

To highlight the infrastructural distinctions between single and dual carriageway bypasses, representative cross-sectional illustrations are provided below (Figure 1 and Figure 2). These images emphasize critical design elements—including lane configuration, median inclusion, and shoulder dimensions—which are essential for assessing their impact on road safety.

4. Results

The first stage of the analysis involved compiling data on traffic accidents recorded on the selected bypasses during the period 2020–2022. To provide a general assessment of traffic safety, the analysis considered both accidents (with casualties) and collisions, with a distinction made between single-carriageway and dual-carriageway bypasses.

The collected data revealed that the number of recorded collisions significantly exceeded the number of accidents across all bypasses. Notably, several bypasses—primarily those serving smaller towns—reported no accidents during the study period. This trend may be partially attributed to the COVID-19 pandemic, which began in 2020 and led to a substantial reduction in vehicular traffic on Polish roads. The highest number of accidents was observed in 2021, likely reflecting a resurgence in traffic volumes following the easing of lockdown restrictions. These findings underscore the limitations of using raw incident counts as the sole metric for evaluating road safety. To obtain a more nuanced understanding, additional indicators such as incident density, concentration, and severity were analyzed.

4.1. Accident Density

Accident density is the number of accidents or collisions per 100 km of road length. It is a basic indicator to determine the level of traffic safety. It serves as a fundamental metric for assessing traffic safety performance. Elevated incident density may result from various contributing factors, including driver behavior, adverse weather conditions, and deficiencies in road infrastructure.

Analyzing accident density enables the identification of high-risk road segments where targeted safety interventions may be necessary. This metric provides a standardized basis for comparing safety levels across bypasses of varying lengths and configurations.

For single-lane bypasses, the average accident density during the period under review was 3.26 accidents per 100 km of road and for dual-lane bypasses it was 5.11 accidents per 100 km of road. Analysis of accident densities on single and dual carriageways suggests that dual carriageways may need to increase road safety measures. Single-lane bypasses, on the other hand, having lower accident densities, may be perceived as safer. It is worth noting, however, that the higher average density on dual-lane bypasses may be the result of higher traffic volumes, for example. Single-lane bypasses appear to have a higher accident density compared to dual-lane bypasses. Their average fluctuates between 74 and 94 collisions per 100 km of road. For dual carriageways, the average fluctuates between 66 and 98 accidents. However, it should be borne in mind that the three-year average collision densities on single-lane and dual-lane bypasses are very similar. They are 81.56 and 79.22 collisions per 100 km of road, respectively (Figure 3).

4.2. Accident Concentration

Accident concentration is the ratio of the number of accidents or crashes to the traffic work performed (the number of traffic accidents per 1 million vehicle kilometers per year). Unlike accident density, concentration allows comparing the level of risk on roads with different traffic volumes regardless of their length.

As can be seen from the charts above (Figure 4), the three-year average accident concentration on dual-lane bypasses is higher than on single-lane bypasses. The analyzed dual-lane bypasses have higher traffic volumes than single-lane bypasses. The higher concentration on these sections may be due to more frequent accidents with heavy traffic and not necessarily indicative of a higher level of danger. The average concentration of collisions was higher on single-lane bypasses. This is interesting given the lower overall number of collisions on these bypasses relative to dual-lane bypasses. The important point here is that single-lane bypasses have much lower traffic volumes compared to dual-lane bypasses, which, with a similar or even lower number of collisions, can result in a higher value of collision concentration.

A high concentration of accidents does not necessarily mean less safety but may be the result of low traffic volumes. In the context of the study, dual carriageways, despite a higher overall number of incidents, may have a higher level of traffic safety.

4.3. Severity of Accidents

Accident severity studies conducted show that the average severity of accidents on single-lane bypasses (15 fatalities per 100 traffic accidents) over a 3-year period is higher than on dual-lane bypasses (11 fatalities per 100 traffic accidents). These results indicate how risk is associated with this type of road infrastructure. The lack of separation of traffic directions or physical separation of them, such as a guardrail at high speeds, significantly affects the level of safety on bypasses.

4.4. Accident Type

The accident type is an important aspect of traffic safety analysis. It allows identification of the most common causes of accidents or collisions. This makes it possible to introduce preventive measures such as the use of additional traffic safety elements, improvements in traffic organization or changes in road infrastructure. The study details several basic types of incidents in Poland: frontal vehicle collision, side vehicle collision, rear-end vehicle collision, vehicle rollover, tree invasion, invading an animal, invading a guardrail, invading an immobilized vehicle, and others. The results are presented in two figures: for accidents (Figure 5) and collisions (Figure 6).

In 2020–2022, on single-lane bypasses, the most common cause of an accident was a head-on collision between vehicles. This is an unavoidable cause of the incident due to the characteristics of the single-lane section. Because of the lack of separation of traffic directions, there is a greater possibility of a head-on collision between vehicles. Immediately after it, on single-carriageway bypasses, the most common causes were side-vehicle collisions and running into a stationary vehicle.

For dual carriageways, the most common cause of an accident turns out to be a rear-end collision between vehicles. When the roadway is bounded by barriers and there is no emergency lane on the road, drivers in an emergency do not have the opportunity to “escape” from the lane, resulting in an increased risk of such an incident. The second most common cause of accidents on dual carriageways was driving into a guardrail. Again, space restrictions may be the cause. On the single-lane bypasses analyzed, the most common cause of collisions was side-vehicle collisions. These incidents most often occurred at single-level intersections and roundabout-type intersections. The second most common type of collision, disregarding the “other” cause, was the invasion of an animal. On dual carriageways, the most common cause of collisions turned out to be invading a guardrail. Immediately after it, without considering the cause “other,” is rear-end vehicle collision.

An analysis of the types of accidents on bypasses shows that the type that occur most frequently is strongly related to the cross section of the road. Different road equipment, the types of intersections used, and space limitations have a significant impact on the possibility of a particular accident. It is also worth noting that not all types of incidents carry the same risk to traffic users. Head-on collisions occurring on single carriageway bypasses are among the most dangerous types of events and are most often associated with high accident severity. In contrast, guardrail invasions and rear-end collisions, while numerous and equally dangerous, usually result in fewer injuries.

4.5. Accident Localization

Determining the location of an incident and, more specifically, the stretch of road where traffic accidents most often occur, is a valuable source of information for designing safe road infrastructure. The study details some of the most common locations where incidents occur: straight section, horizontal curve, slope, elevation, interchange, roundabout, single-level intersection.

Based on the analysis, it can be concluded that the most common place where road accidents occur regardless of the bypass cross section is the straight section. On the analyzed single-lane bypasses, 10 accidents were recorded on straight sections, while as many as 21 accidents were recorded on dual-lane bypasses. The two times higher number of incidents on dual-lane bypasses in straight sections can be attributed to the higher vehicle speeds allowed and the need to change lanes. The second most common place where accidents occur is horizontal curves. On the analyzed ring roads, only one accident was recorded at an intersection—a roundabout-type intersection. It occurred on an exit from a dual carriageway bypass onto a provincial road. The use of a traffic circle as an intersection on a dual carriageway bypass increases the risk of an accident or collision due to the higher speed of vehicles and lower capacity than road junctions. For both single and dual carriageways, the most common place for collisions to occur is the straight section. Out of a total of 549 collisions, 424 collisions or as many as 77% occurred on the straight section. The most common cause of collision occurrence on straight sections is an overtaking maneuver or lane change (for dual carriageways). The second most common place for collisions to occur is horizontal curves. Regardless of the section of the bypass, the most common cause of incidents at these locations is poor visibility and the failure of drivers to adjust their speed to the geometry of the road. On the analyzed single-lane and dual-lane bypasses, a total of 53 collisions occurred at different types of intersections. In total, 41% of them happened at a roundabout-type intersection, with 38% at a single-level intersection and 21% at an interchange. This percentage distribution indicates the high risk associated with the construction of single-level intersections (including traffic circles) as intersections on bypasses. Bypasses are located on major thoroughfares between cities, which results in high traffic volumes. Intersections with other roads should ensure the greatest possible traffic safety. Owing to the absence of at-grade intersections on dual carriageways, the assessment of intersection safety cannot be based solely on the generally higher proportion of recorded collisions at roundabout-type intersections compared to at-grade intersections. To ensure the reliability of safety evaluation studies for these two types of intersections, the analysis was limited to the number of collisions recorded exclusively on single-carriageway bypasses (Figure 7).

In the case of single-lane bypasses, 45% of recorded collisions occurred at at-grade intersections, 41% at roundabout-type intersections, and 14% at grade-separated junctions. Although the highest proportion of collisions was observed at at-grade intersections, the total number of incidents at these and roundabout-type intersections is relatively comparable. The study indicates that, irrespective of the cross-sectional design of the bypass, grade-separated junctions consistently exhibit the lowest number of recorded collisions. This finding supports the conclusion that such junctions represent the safest form of intersection (Figure 8).

4.6. Statistical Analysis

The application of the chi-square test in this study is essential for validating whether the observed differences in incident frequencies across various road configurations and intersection types are statistically significant.

The chi-square test relies on several fundamental assumptions to ensure the validity of its results. First, the data must consist of categorical variables represented by frequency counts, with each observation falling into a single mutually exclusive category. Additionally, the observations should be independent, meaning that the occurrence of one event does not influence another. The test also assumes that the expected frequency in each category is sufficiently large—typically at least five—to allow for reliable approximation of the chi-square distribution. Finally, the data should be derived from a random sampling process to support the generalizability of the findings.

Given that the data consist of categorical variables—such as the type of intersection and the number of head-on collisions—the chi-square test provides a robust framework for comparing the observed and expected frequencies. Its use allows for an objective assessment of whether the variations in safety outcomes are attributable to specific infrastructural features rather than random chance. By confirming statistical significance, the test strengthens the reliability of the findings and supports evidence-based recommendations for road design improvements aimed at enhancing traffic safety.

As part of the statistical significance analysis, a chi-square test was performed to test whether the type of road cross section and the intersections therein significantly affect traffic safety on the study bypasses. A total of five tests were carried out testing a different research hypothesis in each. A summary of the test results is provided in the table. The following questions were asked in the following tests: Test 1—does the cross section of the road affect the number of incidents with victims? Test 2—does the bypass road cross section affect the severity of casualty injuries? Test 3—does the road cross section of the bypass statistically significantly affect the incident rate when average traffic volumes are considered for both types of cross sections? (Detailed data on volumes for each section of the analyzed roads are unavailable.) Test 4—does the type of intersection affect the number of crashes? Test 5—does the bypass cross section affect the type of incidents most often occurring on bypasses? The results of the analyses presented are summarized in the table below (

Table 3. Summary of chi-square results.

	Test 1	Test 2	Test 3	Test 4	Test 5
Degrees of freedom (df)	1	1	1	1	2
χ2 test statistic	1.302	2.007	0.000	2.750	41.420
Critical value α = 0.05	3.841	3.841	3.841	3.841	5.991
Critical value α = 0.1	2.706	2.706	2.706	2.706	4.605
Statistically significant (Yes/No)	No	No	No	Yes (α = 0.1)	Yes

).

The results revealed that the variation in the number of incidents occurring at junctions compared to traffic circles was statistically significant at a significance level of α = 0.1. Additionally, the difference in the frequency of head-on collisions between single-lane and dual-lane bypasses was found to be statistically significant at a significance level of α = 0.5. These findings suggest that both intersection type and carriageway configuration have a measurable impact on road safety outcomes.

5. Discussion

The findings of the analysis clearly demonstrate that traffic safety performance varies depending on the cross-sectional design of bypass roads. Although single-carriageway bypasses recorded fewer total accidents and collisions compared to dual-carriageway bypasses, they exhibited higher incident concentration and severity rates. This indicates that while fewer incidents occur on single-lane bypasses, their consequences—particularly in terms of fatalities and serious injuries—are more severe. These results align with findings from a study conducted in Florida [12], which showed that upgrading single-lane roads to dual-carriageways with a median significantly reduced both the frequency and severity of traffic incidents.

A strong correlation was observed between the type of incident and the road cross section. On single-carriageway bypasses, head-on collisions were the most prevalent, primarily due to the absence of a physical separation between opposing traffic flows. This infrastructure layout encourages risky overtaking maneuvers, thereby increasing the likelihood of frontal impacts. In contrast, dual-carriageway bypasses—typically equipped with central barriers—effectively mitigate this risk. However, these roads were more prone to rear-end collisions and impacts with guardrails, often resulting from limited maneuvering space and the absence of emergency lanes, which restrict drivers’ ability to avoid hazards.

Accident location also played a critical role in safety outcomes. The majority of accidents and collisions occurred on straight road sections. Despite offering good visibility and predictable geometry, these segments often encourage excessive speeds and reduced driver attention, leading to dangerous overtaking and delayed reactions to sudden events. A similar pattern was observed on 2 + 1 cross-section roads, where overtaking lanes are typically located on straight segments. While this configuration improves traffic flow, it can also lead to hazardous maneuvers, particularly near the end of overtaking sections, as evidenced by the Girarduf bypass [8,9].

Intersection type may be another significant factor influencing safety. Most incidents occurred at at-grade intersections and roundabouts. These locations are prone to crashes due to abrupt speed changes, failure to yield, and complex traffic integration. Limited visibility and suboptimal geometric design further exacerbate the risk. Although traffic signals can enhance safety, their effectiveness depends on proper calibration to traffic volumes. The analysis suggests that roundabouts generally offer better safety performance than traditional at-grade intersections, due to their higher capacity, smoother traffic flow, and fewer conflict points. However, it seems that future research should be deepened by entering a slightly lower and more detailed level of analysis of individual types of intersections in order to obtain results that are more statistically different.

Only 21% of accidents occurred at grade-separated interchanges, underscoring their superior safety performance. Interchanges reduce conflict points and facilitate safer turning movements, particularly for left turns and through traffic [13]. Their design also allows for future expansion, making them a flexible and scalable solution for high-volume bypasses. For bypasses with lower traffic volumes, roundabouts or signalized intersections may suffice; however, for high-volume corridors, interchanges are the preferred option from a safety perspective. The result obtained in this study is nothing revealing, but it is worth highlighting, as often, due to the cost aspect, road infrastructure managers abandon the construction of intersections and choose single-level intersection variants, which can worsen the level of safety on designed bypasses.

Overall, the results may indicate the significant influence of both the cross-sectional design and intersection type on traffic safety. When planning new bypasses, it is essential to base design decisions on accurate traffic forecasts and prioritize user safety. The findings support the use of dual-carriageway cross sections and grade-separated interchanges as the most effective configurations for minimizing accident frequency and severity. These insights contribute to the ongoing discourse on optimal bypass design in Poland and similar contexts [10].

6. Limitations

A key limitation of this study is its geographic scope, which is restricted to bypasses located in Poland. Nevertheless, the relatively large sample size provides a robust basis for generalizing the findings within the national context. Due to the high design standards and implementation of safety features on modern bypasses, the number of accidents involving injuries or fatalities was relatively low—an encouraging outcome, but one that limits the statistical power of certain analyses. To address this, traffic collisions were also included in the dataset, although it is acknowledged that not all such incidents are reported in official police records. In addition, the Polish database lacks information on traffic volumes on specific sections of the analyzed bypasses, and the available measurements are often a single section over a distance of several kilometers and several intersections. In addition, measurements are made every 5 years, which makes it impossible to relate the collected data on incidents to traffic volume, and this is an important factor. Despite these constraints, the study offers a comprehensive overview of traffic safety performance on bypasses, differentiated by road cross-section and intersection type. The breadth of the dataset allows for meaningful insights into the relationship between infrastructure design and safety outcomes, providing valuable guidance for future road planning and policy development.