Measuring the Effects of Road Features on Driving Speeds

The Complete Streets approach to roadway design uses site characteristics to create a safer, more accessible environment for all road users. New research demonstrates that roadway design features have a measurable impact on driving speeds. The development of speed reduction factors (SRFs) that estimate the impact of road features on speed reduction provides a data-driven approach to highway design.

To safeguard and accommodate all road users, MnDOT and local agencies have followed the Complete Streets approach when building and converting the state’s roads. The approach includes road design features that have been shown to lower the frequency of collisions and severe injury. Even as agencies are considering new ways to set maximum speed limits, current studies do not fully address how different road features impact vehicle traveling speeds.

Speed limits have conventionally been set based on vehicle interactions with road design and features. The Complete Streets approach emphasizes roadway conditions and speeds that prioritize access to all road users. Field data was needed to determine whether specific road features installed on Minnesota roads may cause changes in driving speeds. The resulting data could provide guidance for future road design and speed limit targets.

What Did We Do?

An initial review of existing research compared studies of various road features that have been deployed to improve performance and accessibility, such as medians, curbs, on-street parking, bicycle facilities and sidewalks.

In the field, potential roadway segments were identified that emphasized certain road characteristics. Road corridors approaching smaller communities provided an opportunity to evaluate driver behavior as vehicles transitioned from higher speed limits on highways to lower speed limits on in-town roads. Also, locations with different roadway types and features allowed researchers to study multiple SRFs. Nineteen roadway corridors were selected, broadly spread throughout Minnesota and encompassing both rural and suburban communities. These corridors included 16 different road characteristics.

“Measuring the effect of different road features on driving speeds provides state and local agencies with helpful guidance to design roads with safety and access for all users,” said Duane Hill, district engineer, MnDOT District 1.

The initial data collection occurred in October 2022, and review of these data prompted another collection period in August 2023. In both collection periods, the study ran during off-peak hours from Monday through Friday. Lidar devices, which use lasers to measure ranges, were deployed at 48 sites to measure the speed and distance of vehicles as they passed through the corridors and encountered speed signage and other road features. The lidar data was transferred to a connected laptop, allowing for real-time recording of all relevant measurements.

Data analysis included statistical correlation between driving speeds and road features. Although posted speed limits remained a factor in the analysis, the study design sought to account for the effect of other speed influencers.

What Did We Learn?

The literature review revealed previous studies on collision and injury reduction correlated to road conversions that sought to increase safety, accessibility and mobility for all users. Collision reductions measured between 19% and 44%, depending on the road features and locations. Although the evidence has pointed to improved traffic safety with Complete Streets guidelines, there is less consensus about principles for setting travel speeds. Fewer studies have collected data on how road conversions and specific safety features affect traveling speeds.

Analysis of speed data collected at the 19 locations demonstrated measurable changes to traveling speeds when drivers encountered various design features. Although this study was not able to compare speeds before and after changes to these design features during road conversions, collecting data across multiple sites allowed for statistical comparison and estimation of SRFs.

Among the road features observed, single-lane roundabouts demonstrated the greatest reduction in average travel speed of 7 mph. Also noteworthy was the 3.1 mph reduction for raised medians. In contrast, depressed medians evidenced a 1.3 mph speed reduction. Other features with noteworthy speed decreases were on-street parking (1.7 mph), crosswalks (1.3 mph) and curbs with gutters (1 mph).

What’s Next?

The demonstrated effect of roadway features on driving speeds provides an opportunity to use the data in designing standards for future road construction and conversion. Other roadway attributes not addressed in this study, such as transit lanes, bus stops, road curves and urban corridors, may also offer promising topics for speed research using a similar study methodology.

Communities continue to express interest in slower, safer roads, and these results can help MnDOT and local agencies to better forecast impacts on speed when using Complete Streets principles to design roads for all users.