SR-91 extends through Santa Ana Canyon between the Santa Ana Mountains and the Chino Hills. Due to the rugged topography in the area, SR-91 is the only east-west freeway between Orange and Riverside counties for dozens of miles. As a result, large volumes of traffic have to travel through a single constrained corridor.
With traffic volumes and congestion levels on the SR-91 freeway increasing sharply in the 1980s, the California Department of Transportation (Caltrans) proposed adding four high occupancy vehicle (HOV) lanes in the median of the highway to increase capacity. Caltrans completed an environmental review process for the project that complied with both national and California requirements. However, after gaining environmental clearance and acquiring the necessary right-of-way, the project stalled amid controversy over the introduction of HOV lanes. As a result, funding reserved for the project from Orange County's Measure M local sales tax devoted to transportation purposes was eventually redirected to other needs.
Caltrans decided to proceed with the project operated as priced managed lanes as the result of a solicitation for candidate P3 projects in response to new state legislation passed in 1989. The decision to develop the project as express lanes prompted an additional state level environmental review focused on the impacts of tolling. This work was completed in 1991 and was followed by two lawsuits. The Cyprus Targa group, a local environment group, sued to stop the project on the grounds that it would have a negative environmental impact, but the court found in favor of the project because it would provide incentives to carpool. Additionally, the Riverside County Transportation Commission filed suit, claiming that the project was unfair because it would be an improvement in Orange County financed with tolls paid by Riverside County residents. The lawsuit was settled out of court allowing the project to proceed.
Construction of the new lanes began in mid-1993 and the new facility opened to traffic in December 1995. The 91 Express Lanes became one of the world's first fully-automated toll facilities, with tolls collected solely by FasTrak™ transponders, and also pioneered the concepts of variable pricing and premium service lanes in the U.S. It was also just the third toll facility implemented on a P3 basis in the Interstate era. The lanes were a major financial success; in the third year of operation, 1998, the CPTC's annual report noted that toll revenues covered all operating costs and all debt service obligations.
Although the lanes were very popular, continued traffic growth in the corridor led to worsening congestion levels on the parallel general purpose lanes. Growing traffic on the express lanes also led CPTC to begin charging HOV 3+ vehicles (those with at least three occupants) to use the facility at a discounted rate. At the same time, a non-compete clause in CPTC's contract prevented Caltrans from making any improvements in the general-purpose lanes. In order to remove the non-compete clause, OCTA ultimately bought out the remainder of CPTC's lease in January 2003 for $207.5 million dollars and took over the operation of the 91 Express Lanes.
In 2014, the Riverside County Transportation Commission initiated a $1.3 billion project to extend the 91 Express Lanes into Riverside County by eight miles from the Orange County line to I-15.