In most countries the traffic rule of keeping right is observed, multi-lane freeways often employ a ru le that requires drivers to drive in the right-most lane unless they are passing another vehicle, in which case they mo ve one lane to the left, pass, and return to their former lane. In order to know whether this rule can effectively pr omote a smooth traffic in both light and heavy traffic, we mainly consider four factors that are traffic flow, safety , rate of lane changing and limited speed. According to car follow ing theory, modified and piecewise segment fixed-en d-beam (FEB) model and hydrodynamic model of mixed traffic , we established highway traffic flow models. Accor ding to the deflection curve of segment fixed-end-beam, we imit ate the overtaking and following process of a vehic le, stretch this process to the whole traffic flow system, imit ate traffic flow by fluid motion, and then create t he freeway model of traffic flow. We gain the solution to the model through the analysis of characteristic lines, the r elations of flow q , designed limit speed f u , density k and rate of lane changing are showed in the follow ing formula: a ce bk q + = , where m b f μ = , ) 1( m k c ff − = μ , m is coefficient of wave velocity, j k is the block density if the speed is zero. We reach a conclusion that the keeping right rule exce pt overtaking can effectively improve a smooth traf fic in a light traffic, but it doesn’t in a heavy traffic. But tra ffic jams often happen in freeways; in this case, t he keeping right rule cannot improve a smooth traffic. To enlarge traffic flow in unit road and enhance safety, we put forwa rd a new rule of limit line and lane after the consideration of s afety distance and maximum traffic. Take a four-lan e freeway as an example, regulations as follows:

1) Limit line: we record the total number of vehicl es running in freeways timely. When the total numbe r reaches the maximum number N , the rule of limit line is employed in the entranc es of freeways, and vehicles are not allowed to enter until the total number of vehicles is less th an N. This rule can guarantee enough safety distanc e between vehicles.

2) Driving in different lanes: the leftmost lanes a re fast lanes, which allow the driving of vehicles with a speed of h km / 120~ 100 . The two middle lanes are ordinary lanes, which al low the driving of vehicles with a speed of h km / 100~ 80 .The rightmost lanes are slow lanes, which allow th e driving of vehicles with a speed of hkm / 80~60 . This rule can enlarge the traffic flow of freeways.