Abstract
Many models were introduced to estimate the roundabout entry capacity from 1980s until now. In the United States, transportation agencies adopted three different models from 1994 until 2010. The Austroads, the UK, and the HCM 2010 methods. In this paper, these three methods were used to analyze the field traffic data of Al-Quds roundabout, located near Baqubah City, simultaneously by utilizing a system dynamics model. The collected data included turning movements, circulating flows, and field calculated entry capacities whenever possible during the observation period which lasted for 14 consecutive hours. A comparison is then conducted on the resulting entry capacities and their variation over time. The results showed that the calculated capacity according to Austroads method is the highest at all times while the UK method was always a little lower and the HCM 2010 method was always the least on all entries. The UK method capacity estimates were the closest to the field measured capacities for they returned the least RMSE on all entries. Field capacities showed some tendency towards the Austroads results in the north and south bound entries which carry about 66% of the total traffic. While, field capacities showed more proximity towards the HCM capacity results in the east and the west bound entries which carry 34% of the total traffic.
Highlights
Many models were introduced to estimate the roundabout entry capacity from 1980s until now
Calculations and Results Analysis The duration of the basic run of the system dynamics model in which all traffic data are fed as exogenous factors, each in due format as mentioned earlier, lasted for a period of hours according to the data collection length and the time step was set to be 0.25 hour to resemble the minute analyses period used in traffic data collection
In all these approach entries, the variable that estimates capacity according to the Highway Capacity Manual (HCM) 2010 method designated as (Cap-1) estimated the lowest capacity value and the Austroads method variable designated as (CAP-3) produced the highest capacity value, while the UK method variable designated as (Cap-2) came in between with a slight tendency towards (Cap-3) in the north bound entry influenced mostly by its geometry which has a great effect on the UK model among the others especially the entry yield line that the north entry has the widest one in the roundabout
Summary
Many models were introduced to estimate the roundabout entry capacity from 1980s until now. The Austroads, the UK, and the HCM 2010 methods In this paper, these three methods were used to analyze the field traffic data of Al-Quds roundabout, located near Baqubah City, simultaneously by utilizing a system dynamics model. The results showed that the calculated capacity according to Austroads method is the highest at all times while the UK method was always a little lower and the HCM 2010 method was always the least on all entries. Field capacities showed some tendency towards the Austroads results in the north and south bound entries which carry about 66% of the total traffic. Field capacities showed more proximity towards the HCM capacity results in the east and the west bound entries which carry 34% of the total traffic
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