A street’s uses, demands, and activities are subject to change over the course of a day. A street at rush hour behaves differently than it does at lunch hour, just as a street late on a Saturday night is used differently on Sunday morning. Street design should be sensitive to how streets operate across all hours of the day, for all users. While understanding peak periods of intensity is valuable, the design of a street or analysis of a corridor should always seek to balance needs and functions of different time periods.
Discussion
Vibrant cities are active 24 hours a day. Streets designed for peak intervals of traffic flow relieve rush-hour congestion, but may fail to provide a safe and attractive environment during other portions of the day. Average Daily Traffic (ADT) and peak volumes alone do not reveal a street’s utilization. Instead, consider multiple hours of travel and average traffic per lane.
Travel time between similar origin and destination pairs tend to be similar across different routes within the network. When one route becomes congested, users choose a different route.1
Urban traffic networks and grids are flexible and resilient due to their inherent connectivity. Design streets from a network perspective, considering turn restrictions and 1-way to 2-way conversions, as well as the overall distribution of congestion throughout the network.
Consider the peak-time activities of pedestrians and bicycles as well as traffic. For pedestrians, peak hours often fall near lunchtime, while bicyclist peak hours typically follow a similar pattern to vehicle traffic, except in cases where demand for greenways or recreational centers peaks on weekends.
Peak congestion conditions are subject to adjustment, as drivers may change their behavior based on expected delay.
A Day In the Life of a Street
AM Peak | 8:00 AM
Signals are adjusted to accommodate rush-hour traffic during the peak hour, metering traffic to prevent gridlock.
Mid-Day | 1:00 PM
Downtown pedestrian volumes reach their peak intensity at lunch hour.
Evening | 8:00 PM
Traffic volumes begin to dip in the evening, after rush hour, while pedestrian traffic in certain areas begins to rise.
Analyze peak points of stress within the overall context and changing use of the street.
Critical
Collect multimodal data over 2–3 hours of peak traffic activity to better understand how traffic behaves through an entire rush-hour period.
Recommended
Residential areas should be designed to enhance the public realm during off-peak hours, while retail corridors may require sidewalk design parameters that accommodate pedestrian flows on weekends and holidays. Transit priority lanes and parking lanes may be governed flexibly throughout the day, with curbside bus lanes being converted to parking on weekends or dedicated loading zones at early morning hours.
Use signal timing or transportation demand management to shift congestion rather than relying upon capacity increases.
Collect 4-hour volumes (AM peak, midday, PM peak, and Saturday) to analyze typical traffic levels. Average these 4 hours and use that volume to guide the design of streets and intersections.2
Utilize performance measures that demonstrate overall corridor travel times as opposed to specific intersection peak level of service only.
Most cities apply ITE’s Trip Generation standards to new developments. Ensure that generated trips are assigned to multiple modes based upon existing mode splits or city adopted mode targets. This reduces additional peak hour vehicle trips generated by the development site and reduces required mitigations.3
Peak-hour parking restrictions for general purpose travel should be limited or converted to other uses. Peak-hour lanes in urban areas, especially those that are directly next to the pedestrian’s path of travel, should be avoided. Peak-hour parking restrictions also limit the use of many other beneficial treatments, such as curb extensions, parklets, and bikeways.
During peak hours, this 4 lane road becomes a 6 lane road, adding 50% more capacity
but reducing opportunities for the 20 nonpeak hours per day.
Analyze streets for all users at both peak and off-peak times to understand their needs and uses within the system. Based on these analyses, explore reallocation and street management tactics, such as temporary pedestrian streets, to better take advantage of the rights-of-way over the course of a single day, week, or year.
Peak-hour analysis has the potential to adversely impact streets in the following ways:
Intersection Design
Warrants for turning traffic often mandate the addition of left- and right-turn lanes to preserve high speeds for through traffic. Reallocate spacing for turn lanes within the existing right-of-way rather than widening the intersection.
Project and Development Review
Traffic impact analysis statements typically require a study of how to accommodate peak-hour volumes. Mitigate peak traffic using operational strategies rather than resorting to increased roadway capacity.
Level of Service (LOS) Calculations
Peak hour volumes are fed into calculation of LOS, which is used to justify costly capacity increases.5
Optional
Restrict parking in favor of high-activity loading zones during morning hours to avoid double-parking on major commercial streets.
Implement combined High-Occupancy Vehicle (HOV)/transit lanes on heavily traveled corridors where HOV traffic would not interfere with transit operations.
To solve peak-hour congestion at one location, look for solutions at the network level. Restricting turns at some locations or removing turn restrictions elsewhere in the grid funnels traffic onto alternate, less-congested routes.
- The phenomenon of traffic evaporation is the flip side of induced demand. When road diets occur, drivers choose an alternate route or even an alternate mode.
S. Cairns, S. Atkins, and P. Goodwin, “Disappearing traffic? The story so far,” Municipal Engineer 151 (2001): 13–22. ↩︎ - DDOT (Washington, D.C.) Comprehensive Transportation Review Manual requires project applicants to collect traffic data from 7–10 AM and 4–7 PM.
DDOT Guidelines for Comprehensive Transportation Review (CTR) Requirements (Washington, D.C.: District Department of Transportation, 2012). ↩︎ - DDOT’s Comprehensive Transportation Review Manual states that any proposed changes to roadway geometry must not add delay to other modes. Project applicants must show how a project affects bicycle, pedestrian, and transit travel.
DDOT Guidelines for Comprehensive Transportation Review (CTR) Requirements (Washington, D.C.: District Department of Transportation, 2012).
The city of Baltimore’s Traffic Impact Study guidelines require that project submissions include counts for pedestrians and cyclists as well as vehicles.
Procedures and Requirements for Conducting a Traffic Impact Study in Baltimore City Pursuant to Ordinance 06–45 (Baltimore: Baltimore City Department of Transportation, 2007). ↩︎ - Data collected by the Pima Association of Governments.
“Annual Traffic Count Program,” Pima Association of Governments, accessed June 3, 2013, http://www.pagnet.org/regionaldata/traveldataandforecasting/annualtrafficcountprogram/tabid/108/default.aspx. ↩︎ - An estimate of the cost of adding a lane, including new curb and sidewalk, to an urban arterial ranges from $1.65 million [Roadway Cost Per Centerline Mile (Tallahassee: Florida Department of Transportation, 2012).] to $4 million [Houston’s Travel Rate Improvement Program: “Toolbox” of Improvement Strategies (College Station: Texas A&M University, 2001).]. ↩︎