Comprehensive evaluation of new integrated freeway ramp control strategy

A new integrated ramp control strategy, recently deployed by the Minnesota Department of Transportation in the Twin Cities metropolitan area, is evaluated. This strategy, stratified zone metering (SZM), takes into account real-time ramp demand and queue size information and aims to strike a balance between two competing objectives: improving freeway efficiency and preventing excessive ramp delays. In this study, the SZM strategy was compared to the earlier ZONE metering strategy as well as to the no-control alternative. Comprehensive metrics were generated through rigorous microsimulation to assess critical aspects of the strategy's performance. The evaluation results are consistent with qualitative field observations and confirm that SZM strategy improves freeway efficiency when compared with the no-control alternative, reduces freeway travel time and delay, improves freeway speed, smooths freeway flow, and reduces the number of stops. More important, excessive queue spillbacks and ramp delays were significantly reduced under SZM control when compared with the earlier ZONE metering strategy. However, the nonrestrictive metering rates under SZM may cause the freeway proper to be overloaded; this leads to compromised freeway performance as compared with the more restrictive ramp control. This is especially true for heavy traffic demands, when higher ramp volumes are allowed on the freeway.