Overview
The State Road and Toll Authority (SRTA) and other state officials have reported increased usage of the recently implemented I-85 HOT lanes in a positive light. However, this does not present a complete picture of the total effect of this system on the entire driving population. SRTA and other transportation officials have been unable (or unwilling) to report in any quantitative way the impact that the implementation has had on the general travel lanes.
Trip times on HOT lanes are expected to be better than travel in the general lanes based on how the system was implemented. However, given SRTA's own recent statistics of 7,000 HOT lane vehicles per day, when compared with the total volume on I-85 at the SR120 monitoring point (aproximately 75,000 vehicles per day in each direction), it's clear that more than 90% of vehicles continue to use the general traffic lanes.
Using the Georgia Department of Transporation's own automatic data collection system, I have analyzed the traffic volume data for I-85 at a collection point just south of SR120 for the periods of time just before and after the conversion of the HOV lanes to HOT (Express) lanes. The collected data shows a significant negative impact on traffic flow during the peak commuting hours.
Data Source
I obtained I-85 traffic volume for two separate three week periods in September and October 2011 using data that is publicly available from the Georgia Department of Transportation's online Traffic Polling and Analysis System (TPAS). This system reports 24-hour traffic data that is collected from Automatic Traffic Recorders (ATRs) located throughout the state of Georgia. In most cases, ATRs are permanent devices that are installed under the surface of the roadways.
Traffic data for the collection point at I-85 just south of SR120 was used. This collection point is within the nearly 16 mile stretch of I-85 which was converted to a HOT (toll) lane configuration from an HOV configuration on 10/1/2011. Vehicle volume data from TPAS for this point is reported on an hourly basis for both the northbound and southbound directions.
Methodology
Only data for the peak Monday - Friday commute days was used. In addition, data for the Labor day holiday (9/5/2011) was excluded since the traffic volume was significantly lower and travel patterns were significantly different on that day. Two separate, three-week reporting periods were sampled, immediately prior to the implementation of the I-85 Express Lanes (9/5/2011 - 9/23/2011) and immediately after the implementation (10/3/2011 - 10/21/2011).
The collected data for for individual hours was averaged for the two separate periods. The resulting numbers show the average vehicle counts per hour during the peak commute days Monday to Friday.
Results
I have made the collected data, calculations and resulting graphs publicly available here (and as a Google Spreadsheet here). The following graphs from that spreadsheet summarize the traffic data for northbound and southbound travel lanes before and after the I-85 Express Lane conversion.
Analysis
As expected, the total average daily volume is nearly identical between the two different time periods (a variance of < 0.2% between the two periods). However, the pattern of traffic is clearly different. For the morning rush, the Southbound travel direction (inbound to Atlanta) has its peak between 7am and 10am and it shows a clear decrease in the number of vehicles during that period of time.
Similarly, for the afternoon rush, the Northbound travel direction (outbound from Atlanta) shows a clear decrease in the number of vehicles during the period of time from 4pm to 7pm.
The graphs also show a slight increase in vehicle counts outside of the peak periods. This indicates that drivers are being forced to alter their schedule due to the inability of the system to handle the total volume during peak hours.
Update 10/27/2011:
The analysis shows a 10.3% reduction in volume on Southbound I-85 at SR120 between 6am and 9am, with an astonishing 17.7% reduction between 7am and 8am - the peak of the morning commute. A similar reduction in volume is seen during the afternoon rush with a decrease of about 6%. This can be explained by the fact that the morning commute traditionally has a much sharper peak than the afternoon commute, and is therefore more significantly affected.
On October 3, 2011 Malika Wilkins with the SRTA stated:
Early projections stated that there will not be a discernible impact to traffic on the general purpose lanes
Chris K. Haley
chris@StopPeachPass.org
