How can variable speed limits improve highway traffic flow?

1 What are VSLs and how do they work?

VSLs are speed limits that change dynamically based on the traffic volume, speed, density, weather, and road conditions. They are displayed on electronic signs that can be updated remotely or automatically by a central system. The system collects real-time data from sensors and cameras along the highway and uses algorithms to determine the optimal speed limit for each segment of the road. The goal is to smooth the traffic flow, reduce speed differences, and prevent shock waves and bottlenecks.

3 What are the challenges of VSLs?

VSLs can come with a range of challenges and limitations, such as high cost and complexity. Implementing VSLs requires a large investment in infrastructure, equipment, and maintenance, as well as a system that can collect, process, and communicate data efficiently and effectively. Additionally, drivers must comply with the speed limit and be aware of any changes. There may be legal and regulatory issues to consider too, such as how to enforce the VSLs, how to handle speed violations, and how to coordinate them with other traffic rules. Furthermore, VSLs may require changes in the legislation, standards, and policies that govern the highway system.

4 How can civil engineers design VSLs?

Civil engineers are essential in designing, implementing, and evaluating VSLs, taking into account various factors such as traffic characteristics, road geometry, weather conditions, user behavior, and system performance. They must apply engineering principles, methods, and tools such as traffic modeling, simulation, optimization, and analysis to create the optimal VSL scheme for each highway segment. The process of designing VSLs includes defining objectives and criteria such as improving traffic flow, safety, or environment; collecting and analyzing data on traffic demand, speed, density, and flow; developing and calibrating a traffic model to simulate the effects of VSLs; designing and optimizing an algorithm to determine speed limits; testing and validating the scheme using simulations or field experiments; evaluating its performance and impacts with indicators like travel time or user satisfaction; and finally implementing and monitoring the scheme on the highway.

5 What are some examples of VSLs?

VSLs have been implemented or tested in several countries, such as Germany, France, Netherlands, UK, USA, Canada, Australia, and Japan. For example, the A9 motorway in Germany uses VSLs to optimize traffic flow and capacity on a 60-km stretch that handles over 100,000 vehicles per day. Similarly, the M25 motorway in UK uses VSLs to reduce congestion and improve safety on a 101-km section that carries over 200,000 vehicles daily. In the USA, the I-35W freeway uses VSLs to improve traffic flow and safety on a 26-km corridor that passes through Minneapolis and has four lanes in each direction while handling over 150,000 vehicles per day.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?