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The Impact of Rush Hour on the 85th Percentile Speed

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Have you ever wondered how much slower we really drive during rush hour? The 85th percentile speed reveals the truth behind typical driving speeds, showing how congestion dramatically affects travel times. From pre-rush to post-rush conditions, the drop in speed can be surprisingly significant, sometimes by as much as 33%! Let’s break down exactly how these fluctuations happen and what they mean for your daily commute.

Understanding the Impact of Rush Hour on Driving Speeds

The 85th percentile speed is a widely used traffic engineering measure that indicates the speed at or below which 85% of all vehicles are travelling on a given road. It provides a reliable estimate of typical driving speeds while mitigating the impact of extreme outliers, such as unusually fast or slow drivers.

A common question is how the 85th percentile speed varies between normal traffic conditions, rush hour, and the pre and post rush hour periods. Let’s explore how congestion influences these speeds and calculate the potential drop in 85th percentile speed during peak hours, as well as how conditions before and after rush hour differ.

Rush Hour Traffic: A Slower Pace

During rush hour, traffic congestion usually results in slower speeds. The volume of vehicles on the road, combined with factors like roadworks, bottlenecks, and traffic signal delays, can cause a significant drop in travel speeds. To understand the extent of this drop, we can calculate the percentage change in 85th percentile speeds.

For example, if the 85th percentile speed is 30 mph during normal traffic conditions, the increase in traffic volume during rush hour could reduce this speed to between 20 mph and 25 mph. To quantify this, we calculate the percentage drop:

If the 85th percentile speed drops from 30 mph to 20 mph: 
Percentage Drop
= 30−2030×100=33.33%
This represents a 33.33% drop in the 85th percentile speed during rush hour.


If the 85th percentile speed drops from 30 mph to 25 mph:
Percentage Drop
= 30−2530×100=16.67%
This represents a 16.67% drop in the 85th percentile speed during rush hour.

Pre-Rush Hour and Post-Rush Hour Conditions

Traffic patterns typically follow a predictable rhythm, with pre-rush hour and post-rush hour periods showing different traffic characteristics from the peak rush hour itself. These times offer important insight into how the 85th percentile speed fluctuates.

Pre-Rush Hour Conditions:

  • Timing: Pre-rush hour conditions typically occur in the early morning, before the major influx of commuter traffic begins. This period usually lasts from 5:00 AM to 7:30 AM, just before the start of the morning rush.
  • Traffic Flow: During this time, roads are generally still clear, with fewer vehicles on the road compared to later in the day. The 85th percentile speed during pre-rush hour is closer to normal or even higher than typical speeds, especially on highways and arterial roads.
  • Example: On a road where the normal 85th percentile speed is 30 mph, during pre-rush hour conditions, it might remain at or slightly above this speed, perhaps 32-35 mph, as traffic volume is low and there are fewer slow-moving vehicles.

Rush Hour: Morning (07:30 – 09:00) and Evening (16:00 – 18:30)

  • Timing: The morning rush hour typically lasts from 07:30 AM to 09:00 AM, while the evening rush hour spans from 16:00 PM to 18:30 PM. These time frames see the highest traffic volumes, with commuters heading to and from work or school.
  • Traffic Flow: During these times, traffic congestion peaks, and travel speeds can drop significantly. Vehicles are slowed by higher volumes of traffic, congestion at intersections, and bottlenecks. The 85th percentile speed will usually decrease by a considerable amount, reflecting the difficulty in maintaining speed due to these factors.
  • Example: If the 85th percentile speed is 30 mph during normal conditions, during morning or evening rush hour, it could drop to 20 mph to 25 mph, reflecting a 16.67% to 33.33% drop, as discussed earlier.

Post-Rush Hour Conditions:

  • Timing: Post-rush hour conditions occur after the morning peak from around 09:00 AM to 11:00 AM, and after the evening peak from 18:30 PM to 20:00 PM. During these times, traffic volume starts to ease as commuters have reached their destinations, and congestion clears up.
  • Traffic Flow: As congestion dissipates, the 85th percentile speed gradually increases, returning to levels closer to the normal or posted speed limits.
  • Example: If the 85th percentile speed was 25 mph during rush hour, it could increase back to 30-35 mph during post-rush hour conditions, as traffic becomes lighter and more fluid.

General Trends in Speed Reduction

The difference in the 85th percentile speed between rush hour and normal traffic conditions can typically range between 16.67% and 33.33%, depending on the severity of congestion. This means that for a road with a normal 85th percentile speed of 30 mph, during rush hour, the 85th percentile speed could drop to anywhere between 20 mph and 25 mph.

This trend holds true across different speeds, with higher-speed roads seeing similar percentage drops. For example:

  • On a road where the 85th percentile speed is 50 mph during normal conditions, the rush hour drop could range between 8.33% (4.17 mph) and 16.67% (8.33 mph), bringing the 85th percentile speed to a range of 41.67 mph to 45.83 mph.

Normal Traffic Conditions: The Impact of Less Congestion

During non-peak hours, when traffic is lighter and more fluid, vehicles are able to travel closer to the posted speed limits or optimal conditions. As a result, the 85th percentile speed is typically higher, often approaching or matching the posted speed limits. In these conditions, congestion is minimal, and the majority of vehicles travel at speeds that align with road design and traffic regulations.

Summary

In summary, the 85th percentile speed fluctuates depending on the time of day and the level of congestion. The pre-rush hour period sees light traffic with speeds close to or above normal conditions, while rush hour traffic results in slower speeds due to increased congestion. The specific rush hour periods of 07:30 to 09:00 AM and 16:00 to 18:30 PM see significant drops in the 85th percentile speed, which can range between 16.67% and 33.33% depending on the severity of the congestion.

Following rush hour, post-rush hour conditions show a gradual return to normal speeds as congestion clears. By understanding these trends, traffic engineers and planners can better assess and predict traffic flow, ensuring more accurate speed limit setting and road safety measures.

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