Can Deerfoot Move Faster if the Speed Limit is Lower at Peak Times?

This week I was interviewed by a number of media outlets regarding Deerfoot Trail. This was spurred by the Province issuing a request for proposals (RFP) to begin planning and design work on three short term improvements that were recommended in May 2017, namely:

  • A McKnight to 64 Ave ramp connection
  • A northbound connection to Deerfoot Trail north of Beddington
  • A Southland Drive to Anderson Road/Bow Bottom Trail southbound basket weave

Along with these construction projects, the RFP also asks the consultant to look at some “intelligent traffic systems” which essentially are ways that Deerfoot Trail could be improved without significant construction, and would instead rely on innovations such as variable speed limits, ramp metering, carpool lanes and variable message boards. The aim of any of these innovations would be to improve commuting times and safety. The media called me with a significant focus on variable speed limits and the underlying question is; Can we really get to our destination faster with slower speed limits?

To answer that, we can look at a study done by a former University of Calgary Engineering student. Karan Arora authored a study, looking at Deerfoot Trail, and found that variable speed limits could remove significant time from on-peak commutes while also significantly improving safety. His study also looked at “Hard Shoulder Running” which would involve opening up the shoulders during on-peak times and leaving them closed for traffic during off-peak hours. By combining variable speed limits with hard shoulder running, Arora found that that average speed on Deerfoot Trail could be increased by 21%, the number of vehicles moving through could increase by 33%, and travel time could be reduced by up to 40%. Additionally, he found that total number of stops during this typical stop-and-go traffic would be reduced by about one third and collisions would be reduced by up to 30%.

He noted that variable speed limits have a number of benefits. By lowering the speed limit, studies have shown that it reduces the differences in speed among vehicles travelling in the same and adjacent lanes. By synchronizing driver behavior, lane changing is discouraged which reduces the probability of a collision, either from a side-swipe or from another driver suddenly slowing down.

Speaking of which, at higher speeds, any small disturbance, such as an abrupt lane change, can lead to a traffic breakdown (meaning the free flow of traffic is halted) which results in stop-and-go traffic. I’m sure you have been driving on Deerfoot while it’s congested and someone all of a sudden darts in front of you, forcing you to make a hard brake. This then causes all of the vehicles behind you to do the same, eventually causing traffic to come to a complete stand still down the line. A big reason for this is because the “critical density” (the maximum density of vehicles for traffic to still flow freely) of the road is lower at high speeds because your following distance needs to be higher in order to react. At slower speeds, the road’s critical density increases as less distance between vehicles is required to travel safely. By increasing the critical density, the traffic breakdown condition can be delayed or avoided. Also, as mentioned in the point above, if traffic is moving in a synchronized manner, lane changes are minimized meaning fewer instances of a possible traffic breakdown.

The study noted that if variable speed limits that are designed to operate proactively, it can result in up to a 39% reduction in travel time. With lower travel times and less stop-and-go traffic, your fuel consumption benefits so you save money by filling up a little less frequently. It would also help with Calgary’s climate change strategy as less greenhouse gas emissions would be caused by cars idling in a traffic jam.

Hard shoulder running, allowing vehicles to travel in the shoulders when road capacity is most needed, could lead to an increase in road capacity as well as average speed. However, the struggle here is the on and off ramps to and from Deerfoot Trail would mean additional lane changes would be required, which as mentioned before, is one of the things a variable speed limit is looking to reduce. On top of that, many studies have shown that adding additional lanes to a road merely induces demand, meaning that extra lanes just draw extra cars to the road resulting in the same level of congestion. I’m hopeful this is still a concept investigated by the province’s eventual consultant.

For those concerned that variable speed limits will only result in slower speeds, it may also increase the speed limit on Deerfoot during off-peak times. The technology available for implementing a variable speed system has improved since other systems across North America and Europe have been installed, using both real-time monitoring as well as time-of-day algorithms. If the system is designed properly, in theory, a variable speed limit system should be able to slow down traffic prior to hitting a bottleneck and making the problem worse.

If you are interested in reading the study by Karan Arora, you can download it here.

This is a provincial matter as Deerfoot Trail remains the Province’s responsibility, but I know many in Ward 12 rely on it every day. If you have any thoughts regarding variable speed limits or other intelligent traffic systems, I’m always open to thoughtful comments and concerns. As always, you can share them with me here.