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Urban Street Design Guide Index

Urban Street Design Guide
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Design Hour

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.
Peak congestion conditions are subject to adjustment, as drivers may change their behavior based on expected delay.
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 hour analysis has the potential to adversely impact streets in the following ways:
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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 on 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

A Day in the Life of a Street

8:00 am

AM PEAK

Signals are adjusted to accommodate rush­hour traffic during the peak hour, metering traffic to prevent gridlock.

1:00 pm

MID-DAY

Downtown pedestrian volumes reach their peak intensity at lunch hour.

8:00 pm

EVENING

Traffic volumes begin to dip in the evening after rush hour, while pedestrian traffic in certain areas begins to rise.

Highest Daily Hourly Vehicle Volume
Analyze peak points of stress within the overall context and changing use of the street.

Critical

Collect multi-modal 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.4
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.
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Washington, D.C. During peak hours, this 4-lane road becomes a 6-lane road, adding 50% more capacity but reducing opportunities for the 20 non-peak 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.
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.

Network Solutions

Click to enlarge.
Turn Restriction
New Pattern

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.

42,000 ADT, Hour by Hour

Broadway Boulevard at Kolb Road in Tucson, AZ is a 6-lane road. 24-hour traffic counts from 2012 illustrate conditions similar to many urban arterials in the country. ADT totals 42,207. Spread across a 24-hour period and accounting for average vehicles per lane, however, the amount of car traffic is well below capacity for nearly all hours of the day.3
Westbound
Eastbound
6-Lane Capacity (700 vpl)
Average Number of Vehicles per Lane (6 lanes)

Footnotes

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  1. 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.

  2. 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).

  3. Data collected by the Pima Association of Governments.

    “Annual Traffic Count Program,” Pima Association of Governments, accessed June 3, 2013.

  4. 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).

  5. 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).].

References

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Adapted from the Urban Street Design Guide, published by Island Press.

References

References for Design Hour: 17 found.