Introduction:

Road safety has become a safety concern worldwide in recent years. According to the world health organization (WHO), road crashes take 8^th place of the total deaths globally between 2000 and 2019 [8]. Addressing road safety issues is a complex problem as there could be more than one factor resulting in a crash [5]. Human, roads, and Vehicles all play their role in the smooth operation of roads. However, it’s interesting that by controlling the road Geometric design, the other two factors can also be managed [1,5]; here comes the concept of forgiving roads, i.e., designing roads that could absorb road users’ errors. This effort is made to think about possible precautionary considerations within the geometric design of roads and to find out various elements of design that have been identified in the research concerning road safety.

Designing safer roads: How can you consider road Safety during the design process?

I would like to highlight some of the features we can consider while designing roads based on my experience in road design and related exposure to road safety audits and interaction with various Clients. Considering the factors below would be helpful in multiple dimensions e.g. reducing chances of driver errors, avoiding the various type of accidents, and ensuring pedestrian safety.

Horizontal Geometry & fitness of road

Big Picture: Carefully study the existing topography and plan smooth geometry within available limitations.

In an endeavor to produce a forgiving design, consider the following:

• Avoid long lengths of straight sections, also avoid sharp & small horizontal curves.
• Avoid unusual geometry such as broken back or reverse curves.
• Horizontal and Vertical geometry should be harmonized.
• Superelevation: should be carefully designed at critical locations such as reverse curves to ensure smooth traffic movement.
• Perform Sight distance and Vehicle turning Analysis for designing safe roads.
• Avoid weaving sections.
• Access points should not be too many, consideration should be given to single entry/exit to avoid weaving sections.
• Footpath route continuity should be provided for pedestrian safety.
• Active traffic Management should be considered in high pedestrian density/demand areas.
• Pedestrian crossings should be designed so that pedestrians are facing opposing traffic while crossing the road/median.
• Pedestrian ramps to be provided at crossing points to cater to wheelchairs and baby strollers.
• Pedestrian guardrail to be considered at critical locations such as crossings.

Temporary Traffic Management: Continuous Pedestrian path should be provided. Safe working width and clear widths should be provided for ongoing construction. Adequate Barriers & safe clearance to be provided in areas of deep excavation. Lane marking to be visible and continuous. Adequate visibility should be provided at crossing points.

Vertical Geometry for smooth roads

• Vertical clearance should be checked & gantries to be provided in advance for bridges, underpasses, and interchanges.
• Very High K values should be avoided (which could induce high speeds).
• Steep gradients should be avoided.
• Roller coaster alignment shall be avoided.
• Alignment should not be spiky.
• Crest curves should have the provision of adequate sight distance.  

Adequate Traffic Signs & Markings

• Provide adequate signage at critical locations such as having animal movement or having chances of landslide.
• Provide rumble strips near roundabouts.
• Consider audible markings at sharp curves or narrow lane widths.

Earthworks/Roadside design for smooth road operation

• Safety barriers should be considered in high fill areas.
• Breast walls & retaining walls to be considered in mountainous terrains at cut/fill locations.

Main elements of Highway Geometry affecting road Safety:

Some of the key geometric design elements that affect the safety of roads are listed below table with a brief description as obtained from research papers:

Geometric Design Element	Effect on Road Safety
Road Carriageway	Carriageway Width: The number of accidents decreases with an increase in carriageway width. However, it is worthy to note that this relation is more evident in developing countries as compared to developed countries. [2,3] To cater to road safety issues, using an optimum lane width from 3.5 to 3.6m has been suggested within the existing literature [5]. However, lane widths greater than 3.7 do not imply such safety enhancement (Transportation Research Board, 1987).   The number of lanes: Modifying the number of lanes has the highest impact on safety and the number of crashes than any other geometric design element. [4]
Shoulder	Shoulder Width: It has been found that safety improves with the increase in shoulder width.   Material: Unpaved shoulders carry less road safety. [2,3]
Median	Median contributes positively to increasing the safety of roads. Width of Median: Directly proportional positively to the safety aspect.   Median Barriers can reduce the cross-over or head-on collisions but can themselves be a cause of an accident, especially concrete barriers; therefore provision of barriers should be provided after proper analysis.
Horizontal Curves	Radii of Curves: Reverse relationship i.e. the small radii could have more safety issues. Curvature: Curvature increases could lead to increased accidents i.e. sharper curves have high chances of road crashes. [2,3,5]
Horizontal Geometry	The number of injury accidents increases if the ratio of the straight section is more and tends to decrease as horizontal curves per Km increase. Similarly, the number of damage accidents increases if roads are straight and decreases as the horizontal curvature increase [1]
No. of Access points	Accident rates increase with the increase in the density of access points due to multiple weaving maneuvers [5].
Grade	Grades: High grades should be avoided as they lead to higher crash rates than milder grades and level geometry. [2,3] It was also observed that the downgrades are riskier than upgrades [5].
Crest Vertical Curves	Sight distances could be an issue on crest curves while vertical geometry is changed. Hence, vertical curves should be carefully designed with proper k values for uninterrupted visibility and increased road safety [7].
Superelevation	Using adequate superelevation can mitigate the chances of severe road crashes (ideally superelevation should be 2% to 3%). [6]
Sight distance	Insufficient sight distance (horizontal and vertical) can increase the chances of road accidents. Adequate Stopping, Intersection, and passing sight distances should be provided in the geometric design of roads. [5]

Conclusion: Key Measures for Road safety in Geometric Design

Based on the above summary, posts on the discussion forum, and the in-class activity, the following could be concluded:

• Measures of road safety shall be considered at the Strategic and Master planning stage to ease Design at later stages;
• Geometric design should be in harmony with the prevailing environment;
• Geometric design should be consistent and smooth;
• Overdesign must be avoided to avoid unusual traffic patterns;
• Number of access points should be adequate to avoid weaving issues;
• Temporary Traffic Management should be well thought off giving adequate provisions for road users, pedestrians, and site working staff;
• Humans, roads, and vehicles should be considered such that the roads are forgiving.

Note: This research work has been produced as part of my routine assignment while doing my Masters in Civil engineering at Qatar University. Course name: CVEN 661 “Geometric Design of Highways”.

References:

[1] A. F. Iyinam, S. Iyinam, and M. Ergun, “Analysis of Relationship Between Highway Safety and Road Geometric Design Elements: Turkish Case.”

[2] Lamm R., Koeckner, J.H. 1980, Safety Evaluation of Highway Design Parameters, Road and Construction, Vol.10, pp. 14-22.

[3] Babkov, V.F., Road Conditions and Traffic Safety, Moscow, 1975.

[4] S. AlKheder, H. Al Gharabally, S. Al Mutairi, and R. Al Mansour, “An Impact study of highway design on casualty and non-casualty traffic accidents,” Injury, vol. 53, no. 2, pp. 463–474, Feb. 2022, doi: 10.1016/J.INJURY.2021.09.042.

[5] “IOP Conference Series: Earth and Environmental Science Relationship of Accident Rates and Road Geometric Design,” doi: 10.1088/1755-1315/357/1/012040.

[6] Zhang Yingxue. (2009) “Analysis of the relation between highway horizontal curve and traffic safety. International Conference on Measuring Technology and Mechatronics Automation, China.

[7] Srinivasan, S. (1982). Effect of roadway elements and environment on road safety. Institution of Engineers, Vol.63

[8] W. H. O. (2018). Global status report on road safety 2018  Retrieved on June 30, 2019 from   In: World Health Organisation [WHO].

Thanks for Feature Image: Photo by Chris Henry on Unsplash