Driver Response to a Dynamic Speed Feedback Sign at a Freeway Exit Ramp Considering the Sign Design and Installation Characteristics

Abstract

Research was conducted at a freeway exit ramp with significant horizontal curvature to evaluate the effectiveness of dynamic speed feedback signs (DSFSs) as a speed reduction countermeasure. Several aspects of the DSFSs were evaluated, including display size, border type, lateral installation position, and vehicle detection range. Three different full-matrix DSFSs were utilized, which included 15-in. display panel with yellow border, 18-in. display panel with yellow border, and 18-in. display panel with no border. Each sign was individually installed and tested at identical locations near the start of the exit ramp curve, in both the traditional right-side-mount and an alternative forward-mount within the exit gore area. Speed data and message activation location were collected for vehicles approaching and entering the curve across the various sign test conditions. Overall, the presence of a DSFS positioned near the start of the curve resulted in curve entry speeds that were, on average, 3.5 mph lower than without a DSFS present. The lowest curve entry speeds were observed for cases in which the message activated when vehicles were within 250 to 400 ft of the curve. Interestingly, earlier message activation did not contribute to further speed reductions, although later activation substantially diminished the speed reduction effects. With regard to DSFS lateral position, both the side-mounted and forward-mounted DSFS installations resulted in similar curve entry speeds. Furthermore, there were no discernable differences in curve entry speeds between the 15- and 18-in. display panels, although the inclusion of a yellow sign border had greater speed reduction effects.

Keywords

operations traffic control devices MUTCD safety signs speed limits traffic signs

Freeway interchange ramps serve as the essential connection between a freeway and another freeway or secondary crossroad. Freeway ramps often include a section of horizontal curvature that typically requires a substantial reduction in speed to be safely negotiated. This curvature often leads to an elevated occurrence of vehicles departing the roadway, resulting in a higher rate of crash occurrence than the adjacent mainline freeway segment ( 1 ). Annually, approximately 700 fatal crashes occur on freeway ramps in the United States ( 2 ). Excessive speed is the most common contributing factor for crashes on freeway ramps ( 3 ). Exit ramps are particularly vulnerable to the effects of excessive speed, owing to the deceleration that must occur when transitioning from the mainline freeway to the ramp, which often contains substantial horizontal curvature. Several prior studies have shown that travel speeds on exit ramps often exceed the exit ramp advisory speed by 8.0 to 21.5 mph ( 4 – 8 ). A recent safety evaluation of freeway exit ramps also found crashes to increase as the speed difference between mainline and ramp speed increased ( 9 ). This speed difference is expected to increase with the trend of increasing speed limits, which often results in an increase in average speed, and an even higher increase in speed by the faster drivers ( 10 , 11 ).

Various traffic control strategies have been implemented to reduce speeds and improve the safety performance on freeway ramps, including chevron signs, ramp advisory speed signs, advance curve warning signs, and rumble strips. However, with the exception of rumble strips, such strategies were not found to be effective speed reduction countermeasures on ramps ( 4 , 5 ). A promising countermeasure to reduce curve entry speeds on freeway interchange ramps is the dynamic speed feedback sign (DSFS). DSFSs utilize radar to detect the speed of approaching vehicles and display targeted warning messages in real time, which may include the actual speed of the approaching vehicle or other messages such as “SLOW DOWN,”“TOO FAST,” or “REDUCE SPEED.” DSFSs have been successfully deployed in work zones, school zones, high-speed arterials, and speed transition zones. However, the use of DSFSs at horizontal curves, particularly on freeway exit ramps, remains limited.

Literature Summary

DSFSs have been used to reduce speeds across various road settings, including school zones, residential neighborhoods, high-speed arterials, speed transition zones, work zones, rural highway horizontal curves, and, more recently, freeway interchange ramps. Across these various contexts, the installation of DSFSs has resulted in the following ranges of average speed reductions:

• 1 to 9 mph in school zones ( 12 – 15 ),

• 1 to 6 mph in residential neighborhoods ( 16 , 17 ),

• 1 to 6 mph on high-speed arterials (12, 18 –23),

• 2 to 6 mph in speed transition zones (12, 15, 24 –26),

• 1 to 4 mph on rural highway horizontal curves (12, 15, 27 –29),

• 2 to 6 mph in rural highway work zones ( 15 , 30 , 31 ), and

• 2 to 10 mph in freeway work zones (15, 32 –37).

Many of these studies also reported substantial improvements in speed limit compliance rates.

In addition to widespread evidence of speed reduction effects across a variety of contexts, DSFSs have also demonstrated crash reduction benefits across a limited number of settings. An empirical Bayes analysis on 192 DSFSs installed in typical municipal settings on arterial and collector roads showed a significant reduction in crashes of all severities and types, ranging from 32.5% to 44.9%, with speed-related crashes demonstrating the greatest reductions ( 38 ). DSFSs have also been well-received by drivers, as evidenced by a survey in California, in which 80% of drivers found the information displayed on the DSFS sign to be useful ( 39 ).

Evaluation of DSFSs at rural highway curves has been relatively limited. A nationwide study by Hallmark et al. investigated the long-term safety and operational effectiveness of DSFSs at 22 horizontal curves ( 28 ). The feedback signs were found to produce a significant reduction in average curve entry speeds of approximately 2 mph, which was sustained during the 2-year study period. The study also found a 5% to 7% reduction in crashes during the first 3 years after DSFS installation. Although the Hallmark et al. study provided evidence that DSFSs are effective for reducing speeds and speed-related crashes on rural highway curves, the effects of DSFSs on interchange ramp curves were not considered, thereby further supporting the need for additional research.

Problem Statement and Objectives

In light of this research gap, in 2018, researchers at Michigan State University embarked on a multiphase research project sponsored by the Michigan Department of Transportation (MDOT) to investigate the effectiveness of DSFSs at freeway exit ramps possessing sharp horizontal curvatures. The initial phases of this research project evaluated driver response to various DSFS messaging alternatives and longitudinal installation positions at four freeway interchange ramps. Compared with the existing site condition (without the DSFS), the DSFS reduced curve entry speeds and improved brake response across most test conditions. Overall, considering the combination of both longitudinal sign position and feedback messaging strategy, the greatest speed reduction effects were attained under the following conditions: 1. the DSFS was positioned at or near the point of curvature (PC) and 2. the feedback message included the driver’s current speed alternating with a “SLOW DOWN” message ( 6 , 7 ). Under these conditions, curve entry speeds were approximately 2 mph lower with the DSFS installed compared with the preinstallation condition. These findings were consistent between the system- and service-interchanges and across all vehicle types ( 7 ). However, the sign was found to have little effect on driver behavior when positioned greater than 350-ft upstream of the PC ( 6 – 8 ), perhaps owing to drivers disregarding such a premature warning message.

Although these evaluations provided encouraging results on the effectiveness of DSFSs at freeway exit ramps, it was necessary to expand the evaluation to include additional sign types, sizes, and installation configurations to provide comprehensive recommendations on the use of DSFSs at freeway exit ramps. To this end, a third and final research phase was undertaken to evaluate the effectiveness of DSFSs as a speed reduction countermeasure in this context considering an expanded set of conditions, which included

• feedback message display size (15- versus 18-in.),

• sign border type (yellow border versus no border),

• sign placement location (right-side-mount versus forward-mount [i.e., gore area]), and

• vehicle detection range.

Field Evaluation

To address these objectives, a field evaluation was conducted at a single freeway exit ramp to evaluate the speed reduction effects associated with various DSFS configurations. Several aspects of the DSFS were tested, including size of the display panel, border type, lateral installation location, and location of each subject vehicle when the sign activated (hereafter referred to as the message activation location). Speed data were collected using LIDAR guns for vehicles approaching and entering the exit ramp curve across the various sign test conditions. A single site was utilized to eliminate site-to-site heterogeneity that would potentially confound the analysis. The following subsections will discuss the various aspects of this evaluation, including test signs, sign installation positions, site description, test conditions, and data collection methods.

Test Signs

Three different speed feedback signs were included in the evaluation, which are displayed in Figure 1. The brightness levels for the message displays were visually matched across the three signs. The general physical characteristics of each sign are noted as follows:

• TraffiCalm sign with a 15-in. full-matrix amber feedback display surrounded by a 40- × 31-in. yellow sheeting border with a black “YOUR SPEED” static text,

• TraffiCalm sign with an 18-in. full-matrix amber feedback display surrounded by a 48- × 36-in. yellow sheeting border with a black “YOUR SPEED” static text, and

• All Traffic Solution (ATS) sign with an 18-in. full-matrix amber feedback display with a white “YOUR SPEED” static panel on top of the display board. This sign did not include an additional border and was consequently a much smaller overall size (30 × 20 in.) compared with the TraffiCalm signs.

The radar systems utilized for vehicular detection also varied between the two sign manufacturers, as follows:

• The two TraffiCalm signs utilized a forward-facing radar with a 30° cone embedded into the sign face.

• The ATS sign utilized a radar unit that was mounted independently from the sign and employed a more focused 10° radar cone, which was intended to better isolate the ramp vehicles and minimize activation of the sign by the mainline vehicles.

The radar units on all signs were calibrated in the field to achieve vehicle detection ranges that were as similar as possible. However, it was not possible to achieve identical performance owing to differences in the radar designs. Specifically, after calibration, the TraffiCalm signs had a typical vehicle detection range (i.e., the location where vehicles would typically be detected by the radar) of approximately 400 ft for passenger cars, which extended to approximately 600 ft for large trucks. However, the ATS sign, with its more concentrated radar band, had a typical vehicle detection range of approximately 600 ft for passenger cars, which extended to approximately 1,000 ft for trucks.

The difference between the vehicle detection ranges of the two radar systems was a crucial variable for this evaluation, as it relates to the location of the approaching vehicle when the feedback message is initially activated, which may affect driver response. Thus, the message activation location was estimated for each subject vehicle and was included as a part of the evaluation. Further information on the collection of this data is included in a later section.

During the field evaluations, the signs were powered using a 140 amp-h portable battery system that was capable of powering the sign for 2 weeks on a single charge. Each of the signs were able to display a variety of speed feedback messages, and could be programmed to display different messages based on the speed of the approaching vehicle. In accordance with the MDOT draft special provision for speed feedback signs, the three signs were programmed to display the following messages:

• speed number only if the approaching vehicle was below 40 mph and

• speed number of the approaching vehicle alternating every 0.5 s with a “SLOW DOWN” for vehicles traveling 40 mph or above.

Sign Installation Positions

A primary condition that was evaluated during the earlier phases of this research project was to vary the longitudinal location of the DSFS with respect to the exit ramp curve. As previously noted, during these initial evaluations the DSFS was consistently found to be most effective when positioned at or near the PC of the ramp curve, with the sign losing effectiveness at distances of 350 ft or greater upstream from the curve ( 6 , 7 ). However, at many exit ramp locations, the typical right-side-mount DSFS installation near the PC is not feasible because of terrain or obstructions, such as a bridge abutment or guardrail, which would potentially block the radar and/or the motorists’ view of the sign.

To that end, a primary variable for the present evaluation was to vary the lateral position of the DSFS with respect to the side of the ramp. Specifically, in addition to the traditional right-side installation at the PC, the DSFS was installed and tested in the ramp gore area, which positioned the sign between the ramp and the mainline freeway and near the green E5-1a exit sign. In this position, the sign was approximately 40 ft beyond the PC, but directly in front of motorists while approaching the curve in the ramp auxiliary lane. This setup is referred to here as the “forward-mount” setup, as displayed in Figure 2a along with the traditional right-side-mount setup (here referred to as “side mount”) example in Figure 2b.

Site Description

A single freeway exit ramp was selected for the field evaluation based on the following criteria:

• Presence of a horizontal curve with an advisory speed of 30 mph or below;

• High frequency of vehicle lane departures at the curve;

• Adequate traffic volumes, while remaining generally uncongested (i.e., minimum average annual daily ramp traffic volume of 1,000 vpd);

• DSFS sign installation capability (i.e., no roadside obstructions or terrain issues); and

• Suitable for data collection.

The selected location was northbound US-127 to Round Lake Road north of Lansing, MI, which is a service interchange with a mainline speed limit of 75 mph and a loop ramp with curve advisory speed of 30 mph. The annual average daily traffic on the mainline is 20,610 and on the exit ramp is 1,332. This site possessed warning signage that was compliant with the Manual on Uniform Traffic Control Devices, including W13-6, E5-1a, and W1-8R signs. A plan view of the existing signage layout and DSFS locations for this site is provided in Figure 3.

Sign Test Sequence

A total of six DSFS conditions were evaluated and compared with the existing site condition without the DSFS. The sign test conditions and corresponding data collection periods were sequenced as follows:

Existing site condition (before DSFS installation);

Side-mounted 18-in. TraffiCalm sign;

Side-mounted 15-in. TraffiCalm sign;

Side-mounted 18-in. ATS sign;

Forward-mounted 18-in. TraffiCalm sign;

Forward-mounted 15-in. TraffiCalm sign;

Forward-mounted 18-in. ATS sign; and

Existing site condition (2 months after DSFS removal).

After collection of data under the existing site condition, the DSFSs were then installed and evaluated according to the sequence shown above. The DSFSs were mounted by MDOT maintenance crews on dual aluminum sign posts, with a 7-ft mounting height from the pavement surface to the bottom of the sign. The existing signage at the site was not modified in any way. The initial DSFS installation remained operational for 7 days before initiating data collection, to allow for dissipation of any driver novelty effects associated with the new traffic control device. For each subsequent change to the DSFS condition, a period of 2 days was allowed to pass before data collection. After completion of all DSFS test conditions, the sign was removed and data were again collected 2-months later under the preexisting site conditions. All data were collected under dry daylight conditions on weekdays (excluding holidays) during December 2020 and February 2021 between 10:00 a.m. and 4:00 p.m.

Data Collection

Vehicle Speeds

Speeds of vehicles exiting the freeway were continuously tracked for the entire exit ramp auxiliary lane, beginning at the start of the taper and continuing to the PC of the ramp curve. A sequence of two handheld LIDAR guns (i.e., police laser) operated by technicians from within separate vehicles parked just beyond the shoulder was used. The LIDAR guns were ProLaser III manufactured by Kustom Signals Inc., which can detect vehicular speed and distance three times per second with an accuracy of ±1 mph at a range of 6,000 ft. From a practical sense, each LIDAR gun is typically only utilized over a range of 1,200 ft owing to geometry and encroachment of other vehicles.

The upstream and downstream LIDAR technicians were positioned 1,350 ft and 500 ft in advance of the PC, respectively. These locations were selected to be away from any critical points (e.g., start of taper, feedback sign, start of the curve) to minimize the influence of the data collection vehicle on drivers. Data were collected from the same location for each test condition. The upstream data collector would track each vehicle at least 100 ft beyond the downstream LIDAR technician, at which point the tracking responsibilities were transferred to the downstream technician, who would track each subject vehicle over the remaining distance to the PC. The data collectors communicated via cellular communications to ensure a seamless “handoff” of the LIDAR speed tracking as each subject vehicle proceeded along the ramp. In doing so, the upstream technician would convey the type and color of each subject vehicle to the downstream LIDAR collector. To isolate driver response to the speed feedback sign, only freely flowing vehicles were included.

Each LIDAR gun was connected to a laptop using a data transfer cable, which allowed for all measurements to be recorded in real time using proprietary software. The computer LIDAR recordings included timestamp, distance, and speed for each measurement. After completion of the LIDAR tracking for each subject vehicle, both data collectors added remarks on the type and color of the vehicle, in addition to any other comments, which were later used to combine the two data sets into a continuous speed profile for each subject vehicle. Collecting data using this LIDAR tracking method provides a significant advantage over cameras or pneumatic tubes, as it provides continuous speed measurements over the entire segment of interest, as opposed to spot speeds at fixed points.

After completion of the LIDAR tracking data collection from the field, both files from the upstream and downstream LIDAR collector were joined using vehicle sequence, type, and color. As the relative distances between the LIDAR collectors and the PC were known, all distances were converted to be relative to the PC. An example representation of the output of this process is shown in Figure 4a. Because LIDAR speeds cannot be measured at the same locations on the roadway for every vehicle, it was necessary to convert this data to a series of spot speeds using an interpolation technique to allow for speeds to be assessed at specific reference points. The combined raw data were linearly interpolated for every foot using the adjacent speeds. Interpolated speeds were then selected at every 50-ft interval starting from the PC, as shown in Figure 4b. Compiling the LIDAR data in this manner provides a robust array of spot speeds at numerous points along the ramp. The data were compiled separately for passenger cars and heavy vehicles (e.g., trucks and buses). Vehicles that were missing large distances during the speed tracking process were excluded.

Feedback Message Activation Location

It is crucially important for the DSFS to be activated when vehicles are at an appropriate distance in advance of the curve to allow enough time and space to react and decelerate accordingly. As noted previously, the vehicle detection ranges varied between the two radar systems, which subsequently affected when the feedback message would activate for approaching vehicles. To assess the relationship between the timing of the message activation and driver response, the location of each subject vehicle on initial display of the feedback message was estimated. These data were collected using an elevated video camera that had been temporarily installed on the roadside during each data collection period. The videos were later reviewed to extract the location of each subject vehicle to the nearest 10 ft using a series of reference markers painted on the shoulder. Because of the variable sign installation locations, for consistency purposes, the location of each subject vehicle was measured from the PC of the ramp curve. The message activation location data were then merged with the LIDAR speed data for each corresponding subject vehicle, and were included as a predictor variable in the subsequent analysis. However, before analyzing the data, the message activation data were first categorized, as follows:

• Less than 250 ft upstream of the PC (late activation);

• 250 to 400 ft upstream of the PC (normal activation); and

• Greater than 400 ft upstream of the PC (early activation).

The 250-ft threshold was selected to represent the approximate braking distance necessary for a vehicle to comfortably decelerate (at 11.2 ft/s²) from 60 mph (the approximate 85th percentile speed at this point) to the curve advisory speed of 30 mph. The 400-ft threshold was selected as it represented the typical passenger vehicle detection range of the TraffiCalm radar.

Data Summary

The speed profile data collected for each test condition were joined, organized, and coded into a single file for a comprehensive statistical analysis. The final data set included complete speed profiles for 2,047 vehicle observations, including 1,983 passenger vehicles and 64 heavy vehicles. Table 1 presents the descriptive statistics, including minimum, maximum, mean, and standard deviation of speed for ramp vehicles approaching and entering the curve. Table 2 displays the frequency distribution of the ramp vehicle data sample by vehicle type, sign position, and message activation location. To check for any obvious trends in the data, sources for potential bias, and data distributions, graphical representations of the data were reviewed and descriptive statistics were compared across each data collection condition.

Table 1.

Descriptive Statistics for Ramp Vehicle Speeds (N = 2,047)

Parameter	Minimum	Maximum	Mean	SD
Speed 1,000-ft upstream of PC, mph	37.61	86.00	66.51	5.92
Speed 400-ft upstream of PC, mph	28.00	76.83	56.23	6.30
Speed 250-ft upstream of PC, mph	27.00	71.43	52.46	5.90
Speed at PC, mph	22.00	58.72	41.19	5.01

Note : PC = point of curvature; SD = standard deviation.

Table 2.

Frequency Distribution of Vehicle Types and Vehicle Detection Ranges Based on Sign Positions (N = 2,047)

Parameter	Message activation location (ft)	Frequency	Percent of sample (%)
Vehicle types (N = 2,047)
Passenger car	NA	1983	96.87
Heavy vehicle	NA	64	3.13
Without DSFS (before sign installation) (n = 330)	NA	330	16.12
15-in. TraffiCalm (n = 188)	<250	127	6.21
	250–400	45	2.20
	>400	16	0.78
18-in. TraffiCalm (n = 151)	<250	55	2.69
	250–400	53	2.59
	>400	43	2.10
18-in. ATS sign (n = 321)	<250	4	0.20
	250–400	37	1.81
	>400	280	13.69
Forward-mounted DSFS
15-in. TraffiCalm (n = 200)	<250	86	4.20
	250–400	51	2.49
	>400	63	3.08
18-in. TraffiCalm (n = 205)	<250	80	3.91
	250–400	78	3.81
	>400	47	2.30
18-in. ATS sign (n = 471)	<250	137	6.70
	250–400	33	1.61
	>400	301	14.71
Without DSFS (2 months after sign removal) (n = 180)	NA	180	8.80

Note: DSFS = dynamic speed feedback sign; ATS = All Traffic Solution; NA = not available.

Data Analysis

Several measures of effectiveness related to vehicle speed were analyzed to determine the effects of the DSFS as a function of sign size/type, lateral installation location, and message activation location. The dependent variables for these analyses were selected to assess driver response to the feedback message and included

• Speed at the PC (i.e., curve entry),

• Speed 250-ft upstream of the PC, and

• Speed 400-ft upstream of the PC.

Additionally, the number of heavy vehicles in the sample was small, which required combining across several test conditions, such that only the DSFS lateral installation position was assessed. All analyses were performed using RStudio. The speed data were analyzed using multiple linear regression, with the form shown in Equation 1,

Y_{i} = β_{0} + β_{1} X_{i 1} + β_{2} X_{i 2} + \dots + β_{k} X_{ik} + ε_{i}

(1)

where

Y_i is the speed at the particular location for vehicle i,

$X_{i 1}$ to $X_{ik}$ are independent variables affecting the dependent variables (including DSFS test condition),

β₀ is an intercept,

β₁ to β_k are estimated regression coefficients for each independent variable, and

$ε_{i}$ is a normally distributed error term with variance $σ^{2}$ .

Results and Discussion

The multiple linear regression results for speeds approaching and entering the exit ramp curve for passenger cars and heavy vehicles are shown in Tables 3 and 4, respectively. Each table presents the parameter estimates and corresponding p-values for speed at three different locations along the approach to the ramp curve, which included speed 400-ft upstream of the PC, speed 250-ft upstream of the PC, and speed at the PC. The parameter estimates displayed in the tables may be directly interpreted as the difference in speed compared with the base condition, which was represented by the existing site before DSFS installation. To assist with visualization of the results, Figure 5 displays the parameter estimates and 95% confidence intervals for “speed at PC” across all DSFS test conditions.

Table 3.

Multiple Linear Regression Results for Passenger Car Speeds Approaching and Entering the Ramp Curve

Parameters	Activation location (ft)	Speed at PC		Speed 250-ft upstream of PC		Speed 400-ft upstream of PC
Parameters	Activation location (ft)	Estimate	p-Value	Estimate	p-Value	Estimate	p-Value
Intercept		19.461	<0.001	8.116	<0.001	0.936	0.342
Upstream speed		0.367	<0.001	0.685	<0.001	0.841	<0.001
Without DSFS (before sign installation)		Base condition
Side-mount DSFS
15-in. TrafffiCalm	<250	−3.290	<0.001	−0.876	0.032	0.401	0.278
	250–400	−4.698	<0.001	−2.431	<0.001	−1.100	0.053
	>400	−3.594	<0.001	−2.443	0.018	−1.430	0.127
18-in. TraffiCalm	<250	−3.040	<0.001	−0.857	0.132	0.513	0.319
	250–400	−4.299	<0.001	−1.600	0.006	−0.054	0.918
	>400	−3.201	<0.001	−1.377	0.035	−0.407	0.493
18-in. ATS sign	<250	−2.676	0.174	−1.703	0.385	−1.481	0.405
	250–400	−4.554	<0.001	−2.065	0.003	−1.050	0.099
	>400	−4.173	<0.001	−2.179	<0.001	−1.365	<0.001
Forward-mount DSFS
15-in. TraffiCalm	<250	−2.738	<0.001	−1.379	0.004	−0.956	0.029
	250–400	−4.663	<0.001	−2.708	<0.001	−1.929	<0.001
	>400	−3.648	<0.001	−2.065	<0.001	−1.566	0.003
18-in. TraffiCalm	<250	−3.538	<0.001	−0.602	0.219	−0.108	0.807
	250–400	−5.247	<0.001	−2.495	<0.001	−1.859	<0.001
	>400	−5.050	<0.001	−2.941	<0.001	−1.877	0.002
18-in. ATS sign	<250	−0.500	0.211	0.009	0.982	0.042	0.908
	250–400	−2.290	0.002	−0.913	0.227	−0.609	0.373
	>400	−3.260	<0.001	−1.347	<0.001	−0.858	0.003
Without DSFS (2 months after sign removal)		0.180	0.625	0.000	0.999	0.344	0.300

Note: DSFS = dynamic speed feedback sign; ATS = All Traffic Solution; PC = point of curvature.

Table 4.

Multiple Linear Regression Results for Heavy Vehicle Speeds Approaching and Entering the Ramp Curve

Parameters	Speed at PC		Speed 250-ft upstream of PC		Speed 400-ft upstream of PC
Parameters	Estimate	p-Value	Estimate	p-Value	Estimate	p-Value
Intercept	8.116	0.086	−4.659	0.278	−8.737	0.039
Upstream speed	0.516	<0.001	0.842	<0.001	0.956	<0.001
Without DSFS	Base condition
Side-mount setup	−4.019	0.010	−0.927	0.509	0.919	0.500
Forward-mount setup	−3.519	0.016	−0.400	0.760	0.338	0.790

Note: DSFS = dynamic speed feedback sign; PC = point of curvature.

Several interesting findings were observed. First, as expected, the speed of vehicles measured 1,000 ft before the curve was strongly correlated with speeds approaching and entering the curve, and this effect was stronger at greater distances upstream of the PC. Specifically, this suggests that faster drivers tended to maintain such behaviors regardless of the DSFS presence at the site and is aligned with prior driver behavioral research.

It is also clear that the DSFSs tended to have a stronger effect on drivers as they proceeded toward the curve. Considering passenger vehicles, speeds measured 400-ft before the curve PC were typically only marginally lower with the DSFS in place. By 250-ft before the PC, the speeds had become consistently lower with the DSFS in place, particularly for the forward-mount setup. The DSFSs showed the greatest effect on speeds measured at the PC (e.g., curve entry point), where speeds were, on average, 3.5 mph lower with a DSFS present. Although the sample of heavy vehicles was small, the DSFSs were similarly effective for reducing the speed of heavy vehicles at the curve entry point. Overall, with the DSFS installed at the site, the average vehicle speed at the PC of the ramp curve was reduced by 7% to 12%, depending on the test condition, compared with the existing site condition. It is also worth noting that vehicle speeds collected 2 months after removal of the DSFS had returned to their preexisting levels. Further discussion of the primary variables of interest, including the effects of lateral sign position, sign size and type, and message activation location are provided in the sections that follow. Please note that hereafter, the discussion will primarily focus on the results pertaining to speeds measured at the PC, owing to the magnitude of the speed reduction effects at this point and that this location represents the start of the critical geometric feature.

Effect of Sign Lateral Position

In general, both lateral DSFS installation positions elicited a similar effect on curve entry speeds. For passenger vehicles, the reduction in curve entry speed ranged between 3.5 and 4.2 mph for the side-mounted setup, and between 2.4 and 4.5 mph for the forward-mounted setup, depending on the sign type and activation location. Although the sample of heavy vehicles was small, speed reductions of 3.5 and 4.0 mph were observed at the curve entry point for the forward- and side-mounted installations, respectively.

Effect of Sign Border and Display Size

An important aspect of this study was to also compare the sign border type (prominent yellow border versus no border) and size of the feedback display (15 versus 18 in.). First, considering the size of the feedback display, the 15- and 18-in. TraffiCalm signs had a similar effect on curve entry speeds. However, there was evidence of a slight interaction effect between the sign border and lateral position. Specifically, the 18-in. TraffiCalm sign had a slightly stronger speed reduction effect than the ATS sign when utilized in the forward-mount position. This may be because of the TraffiCalm’s prominent yellow sign border attracting greater attention with the sign posted in the more visually cluttered gore area. These results align with prior findings evaluating the effect of warning and regulatory signs with enhanced conspicuity on drivers’ curve entry speeds ( 40 , 41 ). Conversely, the ATS sign had a slightly stronger speed reduction effect than the TraffiCalm signs when used in the side-mount condition, which may be a result of the more consistent upstream activation of the sign, which is explained in greater detail in the following section.

Effect of Message Activation Location

The message activation location was found to have the strongest relationship with curve entry speed. The greatest reductions in curve entry speeds were observed for cases where the feedback message initially activated when subject vehicles were within 250 to 400 ft of the curve. For such cases, curve entry speed was approximately 4.5 mph lower compared with cases without the DSFS present. Interestingly, the DSFS was found to be slightly, but consistently, less effective at reducing curve entry speeds when initial activation occurred further than 400 ft upstream of the sign.

Across all test conditions, the DSFS was least effective when the feedback message did not initially activate until vehicles were within 250 ft of the curve. This was probably a result of drivers not being afforded adequate time to react and respond to the message. In such cases, curve entry speeds were 1.3 to 2.5 mph higher than cases where the sign activated for vehicles that were within 250 to 400 ft of the curve. Generally speaking, the effect of sign activation location was dampened when the sign was side-mounted compared with forward-mounted. Late message activation resulted in particularly poor speed reduction performance for the ATS sign when utilized in the forward-mount position. A potential explanation for this may be the lack of a conspicuous sign border, which may have caused the sign to go unnoticed before activation when positioned in the visually cluttered gore area.

Summary, Conclusions, and Recommendations

The initial phases of this research project evaluated driver response to various DSFS messaging alternatives and installation positions at a series of freeway exit ramps with substantial horizontal curvature. Although these prior evaluations provided encouraging results ( 6 , 7 ), it was necessary to evaluate an expanded set of DSFS conditions to develop comprehensive recommendations for their use at freeway exit ramps. To address this objective, a field evaluation was performed that tested several aspects of the DSFS, including physical characteristics (display size and border type), lateral installation location (side-mount versus forward-mount), and vehicle detection range. A single freeway exit ramp site possessing significant horizontal curvature was utilized to eliminate site-to-site heterogeneity that may have potentially confounded the analysis.

Three different full-matrix speed feedback signs were utilized during this field evaluation, which included: 1. TraffiCalm sign with 15-in. display and yellow border, 2. TraffiCalm sign with 18-in. display and yellow border, and 3. ATS sign with 18-in. display and no border. Each sign was installed and tested in both the traditional right-side-mount and an alternative forward-mount (e.g., gore area) position on the roadside. To assess driver response to the DSFS across the various sign test conditions, handheld LIDAR guns were utilized to track the speeds of vehicles approaching and entering the exit ramp curve. The location of each subject vehicle when the feedback message was initially displayed was also collected. The salient results of this field evaluation are summarized as follows.

Compared with the existing site condition, installation of a DSFS near the start of the exit ramp curve resulted in lower vehicle speeds when approaching and entering the curve. Generally speaking, the speed reduction effects of the DSFS increased as the vehicles proceeded toward the curve. The greatest effects were observed at the curve PC (e.g., curve entry point), where overall speeds were 3.5-mph lower, on average, with a DSFS present. Reductions in curve entry speeds were observed across all DSFS test conditions, ranging from 0.5 to 5.2 mph depending on the test condition. Although the sample of heavy vehicles was relatively small, the DSFSs were similarly effective for reducing the speed of heavy vehicles at the curve entry point. It is also worth noting that speeds collected 2-months after removal of the DSFS had returned to prior levels, which further supports the effectiveness of the DSFS as a speed reduction countermeasure at exit ramps.

With regard to the effect of the lateral installation position, both the side-mounted and forward-mounted DSFSs provided a similar level of effectiveness in reducing curve entry speeds, which was consistent for both passenger- and heavy vehicles. Similarly, considering the size of the feedback display, there was no discernable difference in the speed reduction effects between the 15- and 18-in. displays. However, there was evidence of a slight interaction effect between the sign border and lateral position. Specifically, in the forward-mount position, the prominent yellow sign border produced a stronger speed reduction effect compared with no border, perhaps because, with the sign being posted in the more visually cluttered gore area, there was a greater need for conspicuity .

The most interesting results were related to the message activation location, which was found to have the strongest effect on curve entry speed across all of the sign-related variables considered. The DSFSs were most effective when the feedback message was initially activated for vehicles that were between 250 and 400 ft upstream of the curve, and this finding was consistent across nearly all test conditions. For cases in which the feedback message initially activated within this range, curve entry speeds were approximately 4.5 mph lower than without a DSFS present. Interestingly, the DSFS was found to be slightly less effective at reducing curve entry speeds when initial activation occurred for vehicles further than 400 ft upstream of the curve. This finding is consistent with earlier phases of this research project, which found the DSFS to lose effectiveness when installed 350 ft or further upstream from the curve ( 6 , 7 ), as drivers are more likely to disregard speed warning messages when provided too far in advance of the hazard. Note that 350 ft represents the approximate braking distance necessary to decelerate comfortably from 60 mph to the ramp advisory speed of 30 mph, assuming a perception–reaction time of 1.0 s and a deceleration rate of 11.2 ft/s². Not surprisingly, the DSFS was least effective when the feedback message did not activate until the vehicle was within 250 ft of the curve. This diminished effectiveness was likely to be a result of drivers not being afforded adequate time to react and respond to the message.

The findings from this study further confirm DSFSs to be an effective countermeasure for reducing curve entry speeds at freeway exit ramps, and continued use in this context is recommended according to the following conditions. Specifically, the DSFS should be positioned near the start of the curve, with the radar calibrated such that the message activates when vehicles are at least 250 ft in advance of the curve to provide adequate time for drivers to react and decelerate before reaching the curve. In relation to sign size, 15- and 18-in. signs were found to be equally effective and may be used interchangeably at freeway exit ramps. A prominent yellow reflective border around the sign is recommended to help improve conspicuity during cases when the sign is activated late, when the sign is located in a visually cluttered environment, or both.

When necessary, because of obstructions or terrain issues that would otherwise prohibit the traditional right-side-mount, the DSFS may be installed within the gore area of the ramp (i.e., forward-mount setup) between the green exit sign and the initial chevron depending on ramp configuration and geometry. However, caution should be exercised when positioning the sign in the gore area, owing to the increased likelihood that an errant vehicle may collide with the sign in this position. Thus, if possible, installing the sign in the traditional right-side-mount position is preferred.

Although this study involved evaluation of mean curve entry speeds, future research should evaluate whether DSFS has a differing effect on drivers who approach the exit ramp curve at speeds that are considerably lower or higher than average. Furthermore, a long-term evaluation of permanent DSFS installations should be performed to determine whether the effects of the DSFS at freeway exit ramps change with time. Finally, although the current and prior phases of this research project have confirmed the effectiveness of DSFS as a speed reduction countermeasure at exit ramps across a variety of contexts, a future evaluation should assess the effectiveness of permanent DSFS installations of reducing the frequency/severity of ramp lane departure crashes.

Footnotes

Author Contributions

The authors confirm contribution to the paper as follows: study conception and design: T. Gates, P. Savolainen, M. S. Mahmud; data collection: M. S. Mahmud, B. Safaei, T. Gates; analysis and interpretation of results: M. S. Mahmud, T. Gates; draft manuscript preparation: M. S. Mahmud, T. Gates. All authors reviewed the results and approved the final version of the manuscript.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Funding for this research was provided by the Michigan Department of Transportation.

ORCID iDs

Md Shakir Mahmud

Timothy J. Gates

Peter Tarmo Savolainen

Babak Safaei

This publication is disseminated in the interest of information exchange. Michigan Department of Transportation (MDOT) expressly disclaims any liability, of any kind, or for any reason, that might otherwise arise out of any use of this publication or the information or data provided in the publication. MDOT further disclaims any responsibility for typographical errors or accuracy of the information provided or contained within this publication. MDOT makes no warranties or representations whatsoever in relation to the quality, content, completeness, suitability, adequacy, sequence, accuracy or timeliness of the information and data provided, or that the contents represent standards, specifications, or regulations.

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