Guidelines for the Collection of Traffic Calming Data
Traffic Calming Data Collection Guidelines
Step 2: Traffic Calming (Quantitative) Data Collection - Proposed Guidelines
Cut-Through Traffic (Route Modification or Diversion)
High Traffic Speeds (Friction)
Site-Specific Safety Concern (Site-Specific Response)
Step 3: Qualitat ive Data Collection Guidelines
Introduction and Background
In Minnesota, implementing traffic calming techniques is gaining popularity in order to help slow traffic speeds and reduce high volumes of cut-through traffic in residential neighborhoods. However, there are currently no standard guidelines to follow to collect before-and-after data that is used to determine the effectiveness of traffic calming devices. Previous studies indicated the effectiveness of traffic calming devices, but it is difficult to formulate definitive conclusions across a broad range of test sites and projects due to data collection discrepancies. Many times these studies were accompanied with disclaimers because different data collection techniques were used.
For many traffic calming devices, the location and timing of data collection have a significant impact on the results. For example, studies taken very near a speed hump will yield vastly positive results, whereas studies performed further from the device will exhibit much less change in speeds.
Another important variable in data collection is the type of data that should be collected to accurately determine device effectiveness. Since different traffic calming techniques are used to address different traffic problems, the type of data that should be collected will vary based on the device and traffic concern being addressed. For example, a license-plate study is considered an effective way to gather data for problems with cut-through traffic.
In order to determine the effectiveness of a traffic calming device, it is important to accurately compare before-and-after data that is appropriate for the device and to maintain consistency in the location and timing of data collection. Establishing data collection guidelines provides a standard to use in collecting data for different types of devices and consistency in comparing results between various agencies. Therefore, a formalized set of guidelines for before-and-after traffic data collection has been developed, which includes:
- The appropriate location to collect the data
- When the data should be collected
- What measure should be used to collect the data
In addition, a standard for collecting qualitative data (i.e., public surveys) has been developed.
The guidelines were a coordinated effort between SRF Consulting Group, Inc. and various committees that were developed as part of the project. One of the committees involved a focus group that included city, state, academic, and consulting representatives who worked together to develop guidelines. The guidelines were then reviewed by the Technical Advisory Panel for the project.
Members of the focus group included:
| Jim Aamot, Mn/DOT |
Bernie Arseneau, Mn/DOT |
| Jeff Bednar, SRF Consulting Group |
Marc Culver, City of Maple Grove |
| Gary Davis, University of Minnesota |
Russ Matthys, City of Eagan |
| Carla Stueve, SRF Consulting Group |
Joni Giese, SRF Consulting Group |
Objective
The guidelines for collecting traffic calming data were established for use by local agencies to determine the effectiveness of various traffic calming devices. These guidelines provide a unified approach to data collection to better facilitate the consolidation/comparison of findings and to allow the data to be more useful for comparison between agencies, for scientific research, and as information for decision-makers. The guidelines were developed to address various traffic concerns, such as:
Traffic Calming Data Collection Guidelines
Step 1: Identify the Problem
First, study the area to determine the actual traffic problem that needs to be addressed. This will determine the appropriate traffic calming approach to use.
| Problem Definition |
Traffic Calming Approach |
| Route modification or diversion |
|
| Horizontal/vertical realignment and perception techniques (actual and perceived friction) |
|
| Site-specific response (typically, same treatment as speed issues, but an isolated device is used instead of a series) |
Step 2: Traffic Calming (Quantitative) Data Collection - Proposed Guidelines
Cut-Through Traffic (Route Modification or Diversion)
Data Collection Methods
Conduct a before-and-after license-plate survey to determine the amount of cut-through traffic. A study of traffic volumes on adjacent roadways can also be completed to determine if a significant amount of diversion occurred (postinstallation) to roadways that are not designed to handle the increase in traffic.
Data Collection Locations
Entry and exit points to the affected study area.
Timing of Data Collection
Choose the time period with the highest amount of cutthrough traffic. Collect data one month after installation for resident and elected official purposes and again after one year for scientific results.
High Traffic Speeds (Friction)
Data Collection Methods
Several different statistical measures can be used when evaluating speed data. Four of the more significant measures are described below. Since each measure provides different information, the benefits and limitations of each method are also discussed.
Directional speed and volume data is also recommended whenever possible. A study of traffic volumes on adjacent roadways may also be completed to determine if a significant amount of diversion occurred (post-installation) on roadways that are not designed to handle the increased traffic.
85th Percentile Spee d. The 85th percentile speed is a well recognized value that is important in speed studies. This value shows the speed at which 85 percent of the vehicles are traveling at or below. This is also the speed that is typically used to set speed limits, since experience has shown that a speed limit near this value is the maximum safe and reasonable speed for a roadway. This value is most often used to describe a speed study, because it is easy for the public and elected officials to understand. However, this data alone does not give a complete understanding of traffic speeds on a roadway and, in particular, it does not address the percentage of outlying speeds that residents are often concerned with.
10 MPH Pace. The 10-mph pace represents the highest percentage of vehicles traveling in a 10-mph speed range (for example, 25 to 35 mph). Typically, the midpoint of the 10-mph pace corresponds with the average (mean) speed of the roadway. The 10-mph pace can help determine if there is a uniform speed of traffic flow on a roadway, which results in increased safety. However, similar to the 85th percentile speed, this method does not take outlying speed data into account.
Standard Deviation. The standard deviation can be used as a measure of dispersion for traffic speeds. This is often used to understand how tightly the speed data is grouped around the average (mean) vehicle speed. Typically, the standard deviation is the difference between the 85th percentile speed and the average (mean) speed. Standard deviation is useful in determining the uniformity of traffic flow on a roadway. However, another measure, such as the 85th percentile speed or 10-mph pace, is needed to fully understand the speed at which the majority of vehicles are traveling. Since standard deviation is a statistical measure, the general public may not easily understand it.
Percent of Vehicles Traveling 5 MPH Above Posted Speed Limit. This method calculates the percentage of vehicles that are traveling at speeds 5 mph above the posted speed limit. It provides a fairly simple way of understanding the outlying speed data on a roadway and is an easy measure for residents and elected officials to understand.
Data Collection Locations
- Pre-installation: If the location of the traffic calming devices is pre-determined, data should be collected at the assumed mid-point between devices, since traffic calming devices are typically installed in a series. If it is not possible to determine this location, engineering judgment should be used to collect data where peak traffic speeds are occurring.
- Post-installation: Collect speed data at the mid-point between traffic calming devices.
Timing of Data Collection
Collect data for a 48-hour period (ideally, weekdays from Tuesday to Thursday). If data is collected on alternate days, post-installation data should be collected on the same days of the week for consistency. Collect data one month after installation for resident and elected official purposes and again after one year for scientific results.
Site-Specific Safety Concern (Site-Specific Response)
Data Collection Methods
Depending on the perceived problem, various data collection methods could be used. These include point speed at the device; safety data, including crash rate and conflict inventory; and crosswalk compliance checks.
Data Collection Locations
Use engineering judgment to determine where peak traffic speeds are occurring, or collect data at the specific location where the traffic calming device is to be installed, if the location is known.
Timing of Data Collection
- Speed: Collect data for a 48-hour period (ideally, weekdays from Tuesday to Thursday). If data is collected on alternate days, post-installation data should be collected on the same days of the week for consistency. Collect data one month after installation for resident and elected official purposes and again after one year for scientific results.
- Crashes: Crash data for the most current three-year period (using the same consecutive months) should be collected before/after the device is implemented.
- Crosswalk Compliance: Compliance checks should be performed during peak pedestrian usage using the same before-and-after time period. Collect data one month after installation for resident and elected official purposes and again after one year for scientific results.
Note: A sufficient quantity of data is needed for all methods to ensure statistic validity.
Step 3: Qualitative Data Collection Guidelines
We have developed a standard set of questions that can be used to survey residents. These questions can help to determine the perceived problem as well as support for the project. The sample questions include a before (pre-traffic calming installation) and after (post-installation) survey.
Data Collection Methods
Door-to-Door
Door-to-door data collection is considered to have the best response rate, but it also requires a significant amount of time and resources. Due to the high level of effort and the challenge of capturing people at home, this is not considered a feasible alternative for many agencies.
Phone-Calls and Mailings
Mailing surveys to residents is often the most feasible option, but it has been found to produce the lowest response rate. If surveys are mailed, it may be worthwhile to produce a second mailing to those who have not responded and also follow-up with phone calls to the residents that still have not responded.
Data Collection Locations
For most traffic calming installations, the area includes a neighborhood that is easily definable, which is typical for minor collector and local streets. In this case, the agency should use engineering judgment to determine the affected area and locate addresses for the mailing.
If the roadway is a higher classification with a broader travel shed, the agency may consider installing signs that ask for comments by phone. If a larger mailing is used, especially with multiple audiences (for example, businesses and neighborhood residents), the agency may want to consider asking for demographic information to distinguish where the data is coming from. Following are several options for obtaining demographic information:
- Survey Tracking. Assign a number to each survey and keep a key.
- Include a Map. In the survey, include a map with letters or numbers assigned to each area. Ask respondents to identify the area that they reside in.
- Ask Directly. Include a section for personal information, such as name and address. However, since many people are reluctant to share this information, the response rate may be affected.
Timing of Data Collection
The “before” survey should be administered as soon as the agency has collected the appropriate amount of data and is ready to propose a traffic calming measure. The survey should identify where the agency is in the process. For example, the agency has been contacted by concerned residents and completed a preliminary analysis of the area.
Typical “before” surevey questions »
The “after” survey should be sent approximately one month after the traffic calming device has been installed. This will provide adequate time for residents to adjust to the change in the roadway character and for traffic to normalize. However, it will not provide an excessive amount of time so that residents are less likely to respond.
Typical “after” survey questions »
Tips:
- If an agency is in the position to propose a solution, it is beneficial to provide these details to residents. Additionally, if the approximate placement of the devices is pre-determined, it may be a good opportunity to gauge the opinions of the residents who are most directly affected by the device. A map can be included to show the proposed locations of traffic calming devices.
- On “before” and “after” surveys, use open-ended questions carefully. Typically, people do not want to take the time to answer them. However, if this is the first outreach to residents, it can be important to ask open-ended questions (for example, two or three) to gather provide details about the traffic concern.