I've seen people hang rosaries over the rear view mirrors in their cars, and some people wear rosaries around their necks. Is it OK to do that?

First of all, let me give a simple answer and say that I think these practices are fine. I know many devout people who love our Lord and His Blessed Mother who hang rosaries from their cars’ rear view mirrors. For them, it is a way of letting their love for Mary show forth for all to see. The same would be said for those who wear them around their necks. If someone chooses to do either of these practices, they are most likely doing so out of a devotion to and love for our Blessed Mother. While some do not hang the rosary from their mirror or wear it around their neck, they may always have it in their pocket. At night, they may sleep with it wrapped around their wrist. Keeping the rosary close to us is similar to wearing a cross or scapular or hanging a sacred picture in your room. Rosaries are blessed objects and, for that reason, are good to have around.  
 
It must also be said that the Rosary is, first and foremost, an instrument of prayer and one of the best prayers we can pray. The following are some quotes from the saints regarding the rosary.  
 
“Never will anyone who says his Rosary every day be led astray. This is a statement that I would gladly sign with my blood.” St. Louis de Montfort  
 
“Of all prayers the rosary is the most beautiful and the richest in graces…love the Rosary and recite it every day with devotion.” St. Pope Pius X  
 
“How beautiful is the family that recites the Rosary every evening.” St. Pope John Paul II  
 
“There is no surer means of calling down God’s blessings upon the family… than the daily recitation of the Rosary.” Pope Pius XII  
 
“The Rosary is the most excellent form of prayer and the most efficacious means of attaining eternal life. It is the remedy for all our evils, the root of all our blessings. There is no more excellent way of praying.” Pope Leo XIII  
 
“Give me an army saying the Rosary and I will conquer the world.” Pope Blessed Pius IX  
 
"If you desire peace in your hearts, in your homes, and in your country, assemble each evening to recite the Rosary. Let not even one day pass without saying it, no matter how burdened you may be with many cares and labors.” Pope Pius XI  
 
“Our Lady has never refused me a grace through the recitation of the rosary.” St. Pio of Pietrelcina  
 
“The greatest method of praying is to pray the Rosary.” St. Francis de Sales  
 
“One day, through the Rosary and the Scapular, Our Lady will save the world.” St. Dominic