Technical Sheets
This section details the relationship between safety and a variety of road components (horizontal and vertical alignment, road surface, etc.). The principle human capabilities and limitations that analysts must consider in order to gain a clear understanding of the origin of the problems they are facing and to facilitate the choice of appropriate solutions are discussed in greater detail in Chapter 8 of this manual.
Technical Sheet - Horizontal Alignment
The main goal in designing horizontal road alignments is to ensure consistency and uniformity along the road corridor. This approach enhances safety by providing predictability, so drivers can anticipate upcoming road conditions. The ‘no surprises’ philosophy aims to give drivers clear visual cues, such as a good view of an approaching curve, allowing them ample time to adjust their speed accordingly
This technical sheet describes the key characteristics of horizontal curves that need to be considered in a safety analysis:
- Curve radius (or degree of curve)
- Speed differentials.
- Surface condition.
- Superelevation.
- Road width.
- Shoulders.
- Roadsides – sight distance.
- Roadsides – forgiving road
- Passing.
- Warning signs and devices.
- Combination of features.
Technical Sheet - Intersections
Intersections are an essential part of a road network where two or more roads or streets meet or cross: drivers can change their path in them, thus enabling a large number of destinations to be reached with a minimum number of roads.
Intersections are the locations on the network where most crashes occur:
- Conflicts between vehicles and/or vulnerable users are much more likely.
- Human errors are also more likely since users must choose among several paths, adjust their speed and carry out their manoeuvres under high time and space constraints.
This technical sheet doesn’t cover interchanges, while the following conventional types of intersections are presented:
- Uncontrolled and unsignalised (priority) intersections.
- Signalised intersections.
- Roundabouts.
Acceleration and deceleration lanes are also presented in this technical sheet.
Technical Sheet - Road Surface Condition
This technical sheet describes the relationship between safety and two characteristics of the road surface - friction and roughness. There is a strong relationship that has been established between friction and safety.
Technical Sheet - Sight Distance
In a safety study, analysts must determine the available sight distances and compare them with the required sight distances.
Available sight distances are measured at the site. At the design stage, available sight distances may be calculated using graphic or mathematical methods. For existing roads, however, field measurements are recommended as they may reveal sight obstructions that are not shown on plans. These distances may vary significantly according to the selected driver eye height and the height of the object to be perceived. The Sight distance - Technical Study describes how to measure available sight distances; it also indicates the object and eye heights that are recommended in several countries.
This technical sheet describes the underlying principles for determining the required sight distances under various criteria.
Technical Sheet - Vertical Alignment
The main elements to consider when conducting a safety analysis at a grade are:
On downhill grades: increases in braking distances and possibility of heavy vehicle brake overheating.
On uphill grades: speed differentials between passenger cars and heavy vehicles.
On crest curves: restricted sight distances.
On sag curves: water accumulation and accelerated erosion of shoulders due to water run-off.
This technical sheet describes these various problems and how to address them.