SafeCurves

Horizontal curves are one of the most challenging features on high-speed rural roads. The average crash rate on horizontal curves is approximately three times higher than straight sections of road (FHWA, 2023).

Drivers can be surprised by curves that are inconsistent with what they are expecting, elevating the risk of roadway departures and head-on collisions. These types of curves are regarded as ‘out-of-context’ – the sharpness of the curve is out-of-context with the driver’s experience prior to reaching the curve.

Examples of out-of-context curves include:

an isolated curve on an otherwise long and straight road;
the first curve entering a series of curves; and
an unexpectedly sharp curve among a sequence of larger-radii curves.

What causes crashes on horizontal curves?

Research suggests that the primary causes of crashes on horizontal curves include:

complex geometry;
excessive speed;
drivers’ unawareness or misjudgement of the approaching curve; and
environmental conditions, such as visibility or weather.

How might we mitigate crashes on horizontal curves?

There are several low-cost countermeasures which can reduce the risk of drivers leaving the roadway on a curve. These include line markings, curve advisory speeds, warning signs and improving the skid resistance of the road surface. Widening the shoulder and installing roadside barrier also minimizes the severity of a crash, should a loss of control occur.

How can we identify horizontal curves for improvement?

Site analysis

Traditionally, identifying curves for safety improvements involved looking at crash data to find blackspots and trends. Many high-risk curves were only treated after a serious crash had occurred. This approach is reactive, as you need crashes to occur before you can address deficiencies. Chasing crash hotspots fails to address sites where serious crashes could occur in the future. This effect is magnified on low-volume rural roads where crashes are relatively rare and are dispersed across the road network.

Moving to a systemic approach

Rather than waiting for crashes to occur, the systemic approach proactively evaluates risk across an entire roadway system. It is focused on looking for characteristics which are frequently present in severe crash types – for example road geometry or traffic volume. By using a systemic approach, we can identify improvements based not only on crash history but also on crash potential.

Firefly detail of a curve in a road 31812

SafeCurves: Identify

Where does the problem exist?

SafeCurves: Prioritize

What corridors have the most potential for safety improvements?

SafeCurves: Interventions

Where are curve advisory speeds and warning signs required?

Features

Identify locations for improvement - Evaluate curves based on risk factors including speed, geometry, terrain, and superelevation.

Prioritize corridors for investment - Categorize corridors based on risk.
Design countermeasures based on Federal standards - Determine advisory speed and warning signs.

Centerline data - Leverage HERE’s automotive-grade road centerline data.
Integration - View the data using GIS software.

How do you categorize the curves? 

SafeCurves categorizes curves into Class 1, 2 or 3 depending on whether they are:

unacceptable (Class 1)
undesirable (Class 2)
desirable (Class 3)

The design limit is determined by the combination of the approach speed, and the curve radius. For example, a tight radii curve can be desirable if drivers approach it at a low speed, or unacceptable if drivers approach it at high speed.  

The ‘undesirable’ and ‘unacceptable’ design limits are considered out-of-context, with ‘unacceptable’ being more out-of-context than ‘undesirable’.  

How have you validated the data? 

SafeCurves: Identify has been applied to all high-speed ≥40mph roads in the USA covering approximately 213,000 miles of roads and identifying over 1,725,000 horizontal curves.  

The outputs have been validated against data contained in the National Highway Traffic Safety Administration (NHSTA) Fatality Analysis Reporting System (FARS).  

While only 2% of the network was classified as ‘unacceptable’ curves, this category of highway had the highest average number of fatal crashes. The fatal crash rate on ‘unacceptable’ curves is 1.4 times that of ‘desirable’ curves and 2 times that of straights. This suggests that ‘undesirable’ and ‘unacceptable’ curves are high-risk curves. The implication is that targeting interventions at high-risk curves will yield greater road safety benefits and provide a greater return on investment. 

Is SafeCurves available now? Yes. Book a demo or contact us to find out how to access the data for your location. 

How is the service provided?  SafeCurves data is provided via an annual subscription to an access key so that you can use the outputs in your preferred GIS platform.

How will the data be kept up to date? SafeCurves data is updated every 6-months. 

What do you use as your base data? We use HERE centerline.  

The Safe System Approach to saving lives

There are five pillars in the Safe System Approach to road safety. One of the foundational principles in the Safe System is that Safety must be proactive.

We shouldn’t wait for crashes to occur, before addressing risk. Instead, the proactive approach involves using data-driven tools to identify and address potential risks

SafeCurves is the proactive solution to help reduce road trauma on our highways.

Book a demo

Firefly detail of a curve in a road 39405

Who we are

Abley is a specialist New Zealand owned transport planning, road safety and geospatial technology company. Founded in 2003, we work with organizations in New Zealand, Australia, Indonesia, U.S., and Canada across central, local government and large organizations. 

We develop transport strategies and infrastructure solutions that support safer travel, improved accessibility, and greater mobility for our clients and in turn, their customers.   

The products and services that we design leverage our expertise in transport planning, engineering, road safety, location data and geospatial systems.  

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Our approach to managing road safety must change

In 2021, 42,939 people died on America’s roads. A large proportion of these deaths were associated with crashes on horizontal curves.

Did you know the U.S. has more than 400,000 horizontal curves that are out-of-context with the road environment?

What causes crashes on horizontal curves?

How might we mitigate crashes on horizontal curves?

How can we identify horizontal curves for improvement?

Site analysis

Moving to a systemic approach

We need to proactively identify and address risks to save lives. Access to comprehensive data is crucial to achieve this.

This is where SafeCurves comes in.

Learn how SafeCurves can help your organization

Where does the problem exist?

What corridors have the most potential for safety improvements?

Where are curve advisory speeds and warning signs required?

Features

Benefits

Frequently asked questions

Book a Demo

“We would highly recommend SafeCurves to other DOTs because it’s a compelling solution that can significantly benefit a state’s approach to Road Safety.”

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Did you know the U.S. has more than 400,000 horizontal curves
that are out-of-context with the road environment?