FREQUENTLY ASKED TRAFFIC QUESTIONS
1. Won’t a “Children at Play” sign help protect our children?
We often get requests from parents for “Children at Play” signs to be installed in their neighborhoods. Parents hope that the signs will encourage drivers to slow down and drive cautiously. The concern for the safety of children is very important and is shared by highway and street officials. Unfortunately, “Children at Play” signs may not be the best solution. Here’s why:
• Placing signs do not slow vehicles: The speeds people choose to drive along a roadway are generally determined by the roadway characteristics and environment and by the level of comfort and safety the driver perceives. In this type of environment, a reduction in general vehicle speeds through the placement of a “Children at Play” or “Playground” sign should not be expected (especially when the hazard is not consistently clear to the driver).
• Signs are used to warn of consistent, not occasional, conditions: Warning signs are effective when they warn drivers of consistent conditions. Because children are not likely to be consistently playing at a particular location in the neighborhood (unlike at playgrounds or parks), “Children at Play” signs placed there could lose their effectiveness. Studies have shown that when signs are overused or indicate conditions that are not likely and consistent, drivers start ignoring the signs.
• Education and awareness can be important: Even when “Children at Play” signs are used, it may not be a good idea to let your guard down or be lulled into a false sense of safety. Children can benefit from keeping in mind that the street is not a place to play and that not all drivers are necessarily watching out for them.
2. Won’t a lower speed limit lower travel speeds and the number of accidents?
People often believe that posting lower speed limits will force drivers to slow down and will result in fewer traffic crashes. Unfortunately, this is not always true. Here’s why:
• Slow does not mean safe: Although crashes at high speeds can result in more severe injuries, research has found that higher speed limits do not necessarily result in more crashes. The interstate highway system, for example, has the highest speed limits and yet has a very low crash rate when compared with other types of roadways.
• The speed drivers actually travel is often the safest speed: One principle that helps determine a speed limit is that the safest situation on a roadway is when all the vehicles are traveling about the same speed. When a speed limit is set unreasonably low, some drivers tend to ignore the signs. Other drivers will try to obey the posted speed limit. The result is that the roadway is carrying both fast-moving and slow-moving vehicles, which is the perfect recipe for a crash. Strange as it may seem, it is safer for all drivers to be traveling at the same high or low speed than for some drivers to be driving fast and some driving slowly. Therefore, speeds are usually set based on what most people actually drive.
• Other factors: Other factors are also considered in setting speed limits. These include roadway surface conditions, pedestrian and parking activity, and the record of crashes at the location. These factors, however, must be balanced against the hazard of having vehicles on the same roadway travel at very different speeds.
Many people who own commercial property want more driveways to make it convenient for motorists to access their businesses. They may not have considered the negative safety-related impacts of having extra driveways.
• More driveways sometimes result in more crashes: Studies have shown that as the number of driveways along a street increases, so does the number of crashes. Vehicles entering and exiting a driveway cause the rest of traffic—the through traffic—to slow down and sometimes stop. On a street with many driveways, traffic has to slow and stop often to accommodate vehicles pulling in and out of the driveways. This kind of traffic movement can result in more crashes. Reducing the number of driveways and/or creating shared driveways are ways to make traffic flow more smoothly and to reduce the number of crashes.
• The location of driveways is important too: In addition to the number of driveways, the placement of driveways is important. They should be located a safe distance from intersections and where drivers who are entering or exiting the driveway can see clearly for an adequate distance in both directions.
• Limiting driveways doesn’t equal bad business: In recent Iowa case studies, businesses along streets with a limited number of well designed driveways had similar or better retail sales than businesses on streets with more driveways. Motorists generally try to avoid streets where they have to regularly slow down or stop for vehicles that are pulling in and out of driveways. These case study projects are located in Ames, Spencer, Fairfield, Clive, and Ankeny. The retail sales evaluation results for these locations are discussed in the Iowa Access Management Awareness Phase II Report (available from the Center for Transportation Research and Education at Iowa State University).
4. Why aren't there better and longer lasting stripes on the road?
To some motorists it may seem like pavement markings—for example, the stripes on the road that divide lanes of traffic—are reapplied very frequently. The people that apply the stripes to the roads can get pretty frustrated with having to apply and reapply markings. There is no one best material or application technique. Instead, many factors go into the design of pavement markings and the decision of what materials to use and how to apply the markings at each location.
• Durability and cost of materials: When deciding what material to use, durability and cost are evaluated on a case by case basis. Pavement marking materials are generally either paint based or nonpaint based. Paint-based markings are nondurable, lasting from six months to two years. They are relatively inexpensive. Nonpaint-based markings are durable, generally last longer than paint-based markings and are more expensive than paint-based markings. Epoxy is an example of a nonpaint-based marking material..
• Pavement type and condition: The type of pavement surface must also be considered when choosing pavement markings. For example, asphalt concrete surfaces require either hot-applied thermoplastics or heavy applications of paint-based markings in order for the markings to be visible. For streets in good condition, more durable pavement markings are generally used. However, if a street is in poor condition and may be resurfaced soon anyway, one of the less expensive, paint-based materials is usually used.
• Weather and time of year: Paint applied during the summer generally lasts longer than that applied during cold weather. Some nonpaint-based markings are not widely used in Iowa because snow plows have a tendency to remove them.
• Amount and kind of traffic: The more traffic on a street, the more quickly markings deteriorate. Trucks and other heavy vehicles typically wear pavement markings more quickly than do passenger cars. Wear and tear will occur, however, even with common use over a period of time.
5. How are speed limits set? And how do you decide where to put speed limit signs?
Speed limits are generally set at the speed that most drivers travel on a particular roadway. This speed is considered to be reasonable, realistic, and safe. Signs are posted at locations where drivers may need to be alerted to the speed limit.
Speed limits set by what speed drivers actually travel: One principle that helps determine a speed limit is that the safest situation on a roadway is when all the vehicles are traveling about the same speed. When a speed limit is set unreasonably low, some drivers tend to ignore the signs. Other drivers will try to obey the posted speed limit. The result is that the roadway is carrying both fast-moving and slow-moving vehicles, which is the perfect recipe for a crash. Strange as it may seem, it is safer for all drivers to be traveling at the same high or low speed than for some drivers to be driving fast and some driving slowly. Therefore, speeds are usually set based on what most people actually drive.
Other factors: Iowa drivers are expected to know that the speed limit is 20 mph in business districts, 25 mph in residential and school districts, 45 mph in suburban districts, and 55 mph on highways unless otherwise posted. The speed limit of any given segment of roadway may be increased from these standards if it is deemed that a higher speed limit would be more reasonable and safe. Other factors may also be considered in setting speed limits. These include roadway surface conditions, pedestrian and parking activity, and the record of crashes at the location.
Speed limits are posted where they will alert or remind drivers of the speed limit: Speed limit signs must be located at the beginning of a roadway
6. Can all traffic signals be timed so I receive a green light at every intersection?
Because of the complexity of most transportation systems, it would be nearly impossible to orchestrate traffic patterns and traffic signal timing so that everyone could receive green lights at every intersection. Here are some of the factors that determine when traffic signals change and why drivers can’t always get green lights:
Green times vary for minor and major streets: Because major streets carry the largest volumes, traffic signals give a longer green light to the traffic on these streets. This usually minimizes the delay for the majority of the traffic crossing an intersection. However, the smaller volume of traffic on the minor street may sometimes experience larger delays as a result. The minimum total delay is the goal.
Traffic signal timing at isolated intersections: Many traffic signals, especially those at isolated intersections, are designed on an individual basis to change at cycles that are best for that particular intersection. These traffic signal timings are controlled and change either on a pretimed schedule or by adjusting to current traffic conditions. Actuated signals vary their cycle lengths, based on the traffic flow. When a vehicle approaches these intersections, it is detected and the information is sent to an electronic signal controller. The controller adjusts the length of the green light to be optimal for the current traffic conditions.
Some traffic signals are timed as a system: Many signals, especially when closely spaced, are also timed as a system so that some vehicles can travel a segment of roadway and receive a green light at each intersection.
Reducing the delay: In general, traffic signals are timed to reduce the delay for the most vehicles and help traffic flow more smoothly. In a coordinated traffic system, vehicles on the minor street may experience some increased delay since the coordination timing plans favor the higher volume major street.
7. How are signals timed to accommodate pedestrians?
Crossing the street, particularly at a busy intersection, can be dangerous for pedestrians. Traffic signals are designed to make it clear to pedestrians when they can cross and when they should not. Here are some characteristics of pedestrian-related signals:
When WALK symbol is illuminated, pedestrians are advised to check for turning vehicles and then start walking across the roadway. The length of the WALK phase is based upon the minimum start-up time for pedestrians and is typically four to seven seconds.
When DON’T WALK symbol is flashing, it is not safe for pedestrians to start crossing. A common misconception is that pedestrians should not be in the crosswalk when the WALK sign changes to a flashing DON’T WALK sign. However, pedestrians who are already crossing when it starts to flash should have sufficient time to safely finish crossing.
When DON’T WALK symbol is constantly illuminated, it is not safe for pedestrians to start crossing or be in the crosswalk.
• Supplementing the use of signals with caution: Even with the help of these signals, pedestrians should use caution and look for traffic in all directions before crossing.
8. Why can’t we have stop signs to reduce speeding along my street?
We get many complaints from people in residential areas about cars speeding in their neighborhoods. They often ask us to install more stop signs. This concern is very understandable. Unfortunately, adding stop signs may not be the best solution. In fact, you may be surprised to learn, adding stop signs can sometimes make the problem worse. Here is why:
• Stop signs don’t always slow traffic: Strange as it may seem, installing stop signs may not result in reduced traffic speeds. Studies have shown that stop signs are not effective at controlling drivers’ speeds between intersections. In fact, motorists sometimes drive even faster between stop signs to make up for time “lost” while stopped—actually increasing peak speeds and potentially making neighborhoods more dangerous.
• Installing stop signs can do more harm than good: Too many stop signs may also actually discourage good driving habits. Studies have shown that if stop signs are overused or are located where they don’t seem to be necessary, some drivers become careless about stopping at them. This can be especially dangerous for pedestrians and bicyclists who may have a false sense of safety from the existence of a stop sign.
• Other solutions: Fortunately, there are other ways to encourage traffic to slow down. Sometimes even a simple neighborhood awareness program can be effective.
9. Why can’t we have a four-way stop to reduce accidents?
Adding four-way stop signs may seem like it would slow drivers down and make the streets safer, but additional stop signs do not necessarily increase safety. In fact, in some cases, especially when they are not really needed, the overuse of signs can lead to them being ignored by drivers. Therefore, traffic engineers make careful decisions concerning the use of four-way stop signs. Here are some of the factors they consider:
• Too many signs can lead to ineffectiveness: Studies have shown that when stop signs are placed at intersections where they are not really needed, some motorists become careless about stopping. Moreover, overuse of four-way stop signs can contribute to the number of frustrated and impatient drivers on the streets, and these drivers may start driving recklessly.
• Where four-way stop signs are used: Four-way stop signs are often used at the intersection of two roadways that contain similar traffic volumes. The intersection must, however, meet at least one of the following conditions:
» A traffic signal is going to be installed and the intersection needs a temporary solution to control the traffic;
» Within 12 months at least five crashes have occurred at the intersection that could have been prevented by stop signs;
» Relatively high volumes and/or high major-street vehicle speeds exist.
• Other solutions may provide just as much safety: To make travel efficient and safe, four-way stop signs are usually installed only where they are absolutely necessary. Before four-way stop signs are installed, other solutions should be considered. Here are a few examples:
» Relocate the line where vehicles stop to improve visibility at the intersection.
» Limit the number of driveways in close proximity to an intersection since unexpected movements to/from these driveways sometimes cause drivers to suddenly stop or swerve, resulting in crashes.
» Install flashing lights before or at the intersection to warn drivers or to supplement existing stop signs, respectively.
» Install roadway lighting to reduce the frequency of crashes at night.
10. What is the harm in installing an unwarranted traffic control device?
It may surprise you to learn that adding more stop signs or traffic signals along a roadway does not necessarily slow drivers down or increase safety. In fact, in some cases, especially when they are not really needed, the overuse of signs and signals can lead drivers to ignore or not properly obey them.
• Too many signs can lead to ineffectiveness: Studies have shown that when stop signs are placed at intersections where they don’t appear to be needed, motorists become careless about stopping.
• Too many traffic signals can negatively impact traffic flow: Installing traffic signals where they are not needed can create traffic congestion, add travel time, and frustrate drivers, who may start driving impatiently.
• Other options can provide safety: To make travel efficient and safe and to help ensure the proper observance of stop signs and traffic signals, they are usually installed only where they are absolutely necessary. Other solutions—for example, a yield sign—may also provide enough safety, without any detriment to traffic flow.
11. Won’t a traffic signal reduce accidents?
It may surprise you to learn that adding traffic signals would not necessarily increase safety at an intersection. In fact, in some cases, especially when the traffic signals do not seem to be needed, some drivers may begin to ignore them or run yellow lights in an attempt to avoid delays. Therefore, officials in your area make careful decisions concerning the use of traffic signals. Here are some of the factors they consider:
• Too many traffic signals can negatively impact traffic flow: Installing traffic signals where they are not needed can create traffic congestion, add travel time, and frustrate drivers, who may start driving impatiently and make inappropriate decisions. To make travel efficient and safe and to help ensure the proper observance of traffic signals, they are usually installed only where they are absolutely necessary.
• Where traffic signals are installed: At least one of 11 conditions must be met for a traffic signal to be installed. The conditions include high vehicle and/or pedestrian volumes, a record of severe crashes, and school crossings where there is not enough of a gap in traffic flow for children to cross safely.
• Other solutions: Many crashes at intersections are not caused by a lack of a traffic signal. Inexperienced drivers, drunk drivers, and speeding are often the cause. Therefore, traffic signals do not always offer increased safety at an intersection. Other solutions that might be considered include providing turning lanes, installing warning signs, improving roadway lighting, and installing a pedestrian crosswalk.
12. How do you decide where to place signs?
People sometimes wonder why street signs are placed at certain locations and not at others. Once a sign is determined to be necessary, many factors go into the decision of where the sign will be located. Here are some of the principles that determine the most effective location for a sign:
• The type of sign determines where it will be placed: Officials in your area determine the most effective location for the placement of each regulatory sign (such as a “Speed Limit” sign or a “Stop” sign) based on the characteristics of the roadway and the type of information the sign conveys. Warning signs (such as “Narrow Roadway” or “Railroad Crossing Ahead” signs) are posted far enough in advance of a hazard that drivers can see the sign and then respond before reaching the hazard. Guide signs (such as milepost signs and signs that direct motorists to area services) are used where appropriate.
• Drivers expect to see signs on the right side of the road: Most signs are located on the right side of the road. This is where drivers are used to seeing most signs and where they will be looking for signs. Overhead signs may be necessary on expressways or roadways where space is not available or where additional guidance is needed.
• Sign spacing is based on how much time drivers need to read the signs: The spacing between signs is determined by the vehicle speed necessary for drivers to have enough time to read, understand, and make appropriate driving decisions. Too much information too quickly can confuse drivers and result in unsafe decisions and/or actions.
• General principles of sign placement: Street signs are placed only in the public right-of-way of a roadway. Sign should be located where drivers will expect to see signs.
13. Won’t a flashing yellow light draw more attention to a sign?
We sometimes get requests for a flashing light to be added to a sign. It is believed that this would draw drivers’ attention to a sign and encourage them to slow down. You may be surprised to learn that this is not always true. Traffic engineers study and evaluate what locations would benefit from flashing lights and at what locations flashing lights would be a detriment. Here are some of their findings:
• Flashing lights are used to alert drivers to conditions that are unusual and/or unforeseeable: Flashing lights should be used to warn drivers of unusual conditions that are not readily apparent and that require more than normal care—for example, when there is an obstruction in the roadway ahead or a narrow bridge around a curve. Some flashing lights are effective in high-speed rural environments where the roadway suddenly changes, at locations where pedestrians frequently cross, and along roadways that experience a large number of drivers who are unfamiliar with the roadway. If conditions are not unusual or unforeseeable, flashing lights may not be necessary or make the roadway any safer.
• When flashing lights are overused, they may become ineffective: The effectiveness of flashing lights at any given location decreases with time. Studies have shown that when flashing lights are used too often, some drivers stop paying attention to them. Drivers may even start to ignore important signs that aren’t supplemented by flashing lights.
• Existing signs are often adequate: Unless conditions are unusual and/or unforeseeable, existing unlighted signs are often effective in conveying their message to drivers.