In this paper, we present a new macro model for traffic flow on a highway with ramps based on the existingmodels. We use the new model to study the effects of on-off-ramp on the main road traffic during the morning rush periodand the evening rush period. Numerical tests show that, during the two rush periods, these effects are often differentand related to the status of the main road traffic. If the main road traffic flow is uniform, then ramps always producestop-and-go traffic when the main road density is between two critical values, and ramps have little effect on the mainroad traffic when the main road density is less than the smaller critical value or greater than the larger critical value. Ifa small perturbation appears on the main road, ramp may lead to stop-and-go traffic, or relieve or even eliminate thestop-and-go traffic, under different circumstances. These results are consistent with real traffic, which shows that thenew model is reasonable. In this paper, we present a new macro model for traffic flow on a highway with ramps based on the existing models. We use the new model to study the effects of on-off-ramp on the main road traffic during the rush rush and the evening rush period. Numerical tests show that, during the two rush periods, these effects are often different and related to the status of the main road traffic. If the main road traffic flow is uniform, then ramps always produce stop-and-go traffic when the main road density is between two critical values, and ramps have little effect on the main road traffic when the main road density is less than the smaller critical value or greater than the smaller critical value. Ifa small perturbation appears on the main road, ramp may lead to stop-and-go traffic, or relieve or even eliminate thestop-and-go traffic, under different circumstances. These results are consistent with real traffic, which shows that the new model is reasonable.