Blog skyhook logo From Bad to Worse: Boston Traffic Negatively Impacted by HOV Lane Change on I-93

Aug 16, 2019 11:00:00 AM

From Bad to Worse: Boston Traffic Negatively Impacted by HOV Lane Change on I-93

Posted by Melissa Burke

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If you’ve ever had to commute into the Greater Boston area during rush hour, you understand why Boston is described as the most congested city in the U.S. Ask any Bostonian and you’ll hear at least one story of how what should have been a 20 minute commute turned into over an hour trip somewhere. With this uncertainty and unpredictability, Boston commuters have to build more time into their daily commute and plan for the worst case scenario, which happens way more often than it should. 
As a Boston-based location data and technology company, we wanted to use data to investigate if traffic had really gotten worse, and if opening up the high-occupancy vehicle (HOV) lane on I-93 to every commuter had an impact on the traffic flow.

HOV to “Express Lane”

In May 2019, the Massachusetts Department of Transportation, in an effort to lessen congestion specifically during a large construction project on the Tobin Bridge, renamed the HOV lane between Medford and the Zakim Bridge on I-93 the “express lane. This move opened it up to all commuters, not just carpoolers, with the goal of improving the commute and easing congestion. 

The Analysis

We analyzed location data from anonymous mobile devices during the weekday morning commute of I-93 in Boston on the stretch of highway between Medford (intersection with route 16) and the O’Neill Tunnel between the hours of 7am and 10am. We looked at the week of June 25-29, 2018 as the control period and compared it to June 24-28, 2019, about a month after the HOV lane was opened to everyone. 
This data gives us unique insight into how many people vs. cars were moving into the city and thus accounting for single and multi-passenger cars.This is important because the city’s goal is to move as many people into the city and not vehicle

The Result: HOV Removal Doesn't Get Commuters Into the City Efficiently 

When analyzing the data of mobile devices traveling on I-93 year over year, we found that congestion has become noticeably worse. The amount of time spent in traffic was higher and the average speed of cars traveling was slower. This infers that opening up the lane to all commuters has made the traffic worse and has made the commute time longer for previous carpoolers. 
Here are some other key findings:
Since in June 2018 the HOV lane was limited to only a portion of the commuters deemed as “carpoolers”, we’ve inferred that that select group would have been traveling at speeds higher than those in the more congested all-traffic lanes. Therefore, we focused on the distribution of the average speed of the commuters.  
    • The closing of the HOV lane especially affects those who carpool. This group experienced a dramatic 53% drop in their average speed.
The elimination of the HOV lane was intended to reduce congestion by more evenly distributing traffic between all available lanes. Based on this assumption, one would expect that even if the average speed of travel would drop, the median would increase, signifying that half of the commuters drove faster in June 2019 than in June 2018. 
Indeed, we did see the standard deviation drop from 9.7 to 9.1, indicating that there is now less variation in average speed between all drivers. However, what our data shows is an 8% drop in the average speed of travel, and 0.4% drop in the median. 
    • This in turn means that the predicted 50% of commuters did not experience the desired increase in average speed. 

Potential Unintended Environmental Consequences 

HOV lanes carry more passengers per hour than general-purpose highway lanes, and they are created in part to help with Massachusetts dual goals of reducing greenhouse gas emissions from traffic and easing congestion, Chris Dempsey, director of the advocacy group Transportation for Massachusetts noted. According to the state, currently about 45% of Massachusetts carbon emissions are coming from transportation. Opening up the HOV lane to everyone removes the incentive for people to carpool, which not only makes the traffic worse but may cause a negative effect on the environment as well. 

Informing Your Work from Home Days

One benefit from running this analysis was an unintentional insight into which days of the week have the slowest traffic and therefore the longest commutes. Of all the weekday commutes, the greatest drop in average speed was seen on Tuesday and Thursday mornings, perhaps demonstrating which days would be best for Boston employees to work from home and skip the commute. If your company doesn’t currently have a work-from-home policy, don’t worry. In a recent report to the Governor, Congestion in the Commonwealth, it was said that the city of Boston wants to work with Boston employers to incentivize working remotely as one of the solutions to ease the congestion.
Average MPH for Boston Commuters between 7:00AM and 10:00AM on I-93 stretch from Exit 16- O’Neill Bridge



The change from HOV lanes to “express lanes” was intended to make traffic improve, but this data shows that not only did the traffic not improve, people were spending longer in traffic compared to the previous year. The data provides a unique perspective on the issue and can be used by the City of Boston as evidence needed to support a decision on whether or not to keep the lane open to everyone. We hope this provides some much needed insight on the matter. 
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