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12 with a curve arrow and a speed advisory plate of 45 mph is proportionately lower speeds. For example, for the two "higher ranked" than a curve with a turn arrow but no advi- sequences with the lowest speed advisories of 20 and 25 mph, sory speed plate. the daytime average speeds were 10 to 11 mph over the advi- By way of context, the values of the curve hierarchy for the sory, nighttime averages were 7 to 8 mph over, and the MSP DPM sequences ranged from 1--a curve with no curve-related averages were 12 and 16 mph over. For the four sequences treatments--to 17--a complex curve with curve warning with 35-mph speed advisories, daytime averages were 5 to signs, advisory speed plaques, and chevrons. In some instances, 8 mph over, nighttime averages were 5 to 7 mph over, and there are multiple curves in a DPM sequence although they the MSP averages were 11 to 15 mph over. For the sequence are not necessarily reverse curves. The individual curves in with the 40-mph advisory, the daytime average was 1 mph a single DPM sequence may, and frequently do, have differ- over, nighttime was at the advisory, and the MSP average ent values within the hierarchy. Finally, the sequences con- was 7 mph over. For Sequence 1 where the second curve in tain situations in which the worst curve was the first one the sequence had an advisory of 40 mph, daytime drivers encountered and other situations in which the worst curve were 6 mph over, nighttime drivers about 3 mph over, and was the second or third one encountered. In terms of obser- MSP officers 11 mph over. Not considering the sequences on vations, there were 42 drivers including the 3 MSP officers. gravel roads, for the sequences with no advisory speeds and The number of drivers having one or more errors at the var- regulatory limit 50 or 55 mph, the daytime average ranged ious sequences ranges from 1 to 35. With one exception, all from 47 to 52 mph, nighttime from 44 to 48 mph, and the curve sequences presented problems to almost a quarter of all MSP from 52 to 57 mph. drivers; the "worst" sequences resulted in at least one error Based on these results, it is noted that while drivers slow assigned to almost 90% of the regular (non-MSP) drivers. down when confronted with speed advisories and slow down even more for lower advisories, they are still more likely to exceed lower advisories by a greater amount than they do Discussion of DPM Results higher advisories--their speed-reduction response is some- Given the DPM results for individual sequences, the ques- what limited. There are several possible explanations for this: tion is whether there are overarching issues that emerge and, drivers may feel that the risk of exceeding the advisory speed especially, whether there is anything that may impact the is lower at lower speeds; they may simply feel that they don't guidelines for the use of curve signs and markings. In the dis- need to go that slowly; or they may perceive that they have cussion that follows, reference is made to specific DPM or slowed down the requisite amount--that is, their sense of curve sequences. speed may be diminished. The latter explanation is similar to the reasoning for freeway off-ramp crashes where the appro- priate exit speed, sometimes as low as 25 mph, is misjudged Advisory Speeds in comparison with the mainline freeway speed, which can be 70 mph or higher--drivers are simply not going as slow Perhaps the most interesting finding that emerges from the as they think they are. sequence-based results is the response of drivers to advisory Assuming that speed advisories are marked using the speed signs and to regulatory speed limits. First, there were MUTCD as a general guide, drivers are consistently driving three different speed limits encountered by drivers on the faster than the curves are marked. Moreover, the MSP speeds DPM route: 45, 50, and 55 mph. In addition, there were two indicate that for most situations, the maximum safe speed is gravel-road sequences where the speed limit was unmarked even higher. However, for curves with the lowest advisory and, thus, a de facto 55-mph limit was in effect (Sequences 4 speeds, the number of drivers with curve-related errors is the and 6). Signed advisory speeds for the DPM sequences ranged greatest over all sequences (25 and 24 of 39 regular subjects, from 20 to 40 mph. Speed advisories were not present at all respectively). curve sequences, and none was present on either sequence on Based on the results just presented, the question arises of the gravel roads. On the curve sequences, the average speeds what would happen if the existing guidelines for speed advi- of the regular drivers never exceeded the posted or de facto sories were to be changed, resulting in advisories being in- speed limits. On the gravel road segments, regular drivers creased. While there are different scenarios that might occur, were quite conservative, approaching curves at average speeds the worst-case is that drivers maintain the difference between less than 35 mph. The MSP officers drove similarly, although posted advisories and their travel speed. Under this scenario, faster, in all instances except on the curve sequences on gravel if the curves with 20- and 25-mph advisories on the DPM roads where they drove about 2 mph slower than the other route were increased 5 or 10 mph and drivers continued to drivers. exceed these lowest advisories by 10 to 11 mph in the day- Considering those sequences marked with advisory speeds, time, they would be negotiating the curves at 35 and 40 mph in every instance drivers drove, on average, faster than the or similar to the speeds of the MSP officers. An increase in speed advisories that were displayed. Moreover, there is curve-related driver errors could be expected. Under this sce- some indication that lower speed advisories did not result in nario, where existing advisories were 35 mph and drivers
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13 were 5 to 8 mph above the advisory, if these advisories were "problem" within the context of DPM, it seems clear that the raised to 40 mph, drivers could be expected to drive at 45 to problematic sequences that emerge are those that require sig- 48 mph--only a little under the MSP speeds of 46 to 50 mph. nificant speed reduction at the outset and those where the It remains to be seen how drivers would respond to increas- necessary reduction, although less, is within a sequence of ing advisory speeds, but the evidence here suggests that there curves. could be problems, especially at those curves requiring the The recommendation for the sequences where the initial lowest advisories. speed is relatively high but decreases at a subsequent curve The findings with respect to drivers exceeding speed advi- within a series of curves is to continue to sign the lower- sories is consistent with findings from other parts of this proj- speed curve aggressively in spite of the number of drivers ect where drivers reported that they consistently exceed making errors even when speed advisories and other treat- posted advisory speeds at curves and where practitioners ments are present. This recommendation is based more on indicated that their expectations and perception of common common sense and judgment than the study results per se. practice were that drivers routinely exceeded advisories. In terms of recommendations, what emerges from this analysis, at least in the short term, is that raising advisory Driver Errors speeds may well result in problems for drivers who routinely exceed advisories. Moreover, the problem could be worse on The sequences were also examined by ordering them curves with the lowest advisories. Thus, the effects of driver according to the number of drivers having errors. In addition response to raised advisories should be thoroughly studied to speed-related errors at the curves with the lowest advi- before changes are made. Given the numbers and types of sories, the greatest number of drivers (24 and 25) making driver errors with existing treatments, it seems appropriate non-intersection errors occurred for these same curves. The that signing for curves with the greatest difference between next highest numbers of drivers having non-intersection the posted speed limit and the advisory speed should be com- errors occurred for a curve with a 35-mph advisory (19) and prehensive (e.g., advance warning signs, speed advisories, two that had no advisory (18 and 19), one of which had a chevrons, or centerline markings). While the use of compre- posted limit and one of which was not posted. Thus, looking hensive treatments did not result in lower numbers of driver at the speeds and the driver errors, for at least some curves errors in the study, drivers should still be given as much in- what happens is that speeds in excess of the advisories, and formation as possible on curves where the desired speed re- especially the lower advisories, lead to greater numbers of duction is the greatest. driver errors. Examining the driver errors in more detail, an arbitrary distinction between sequences where 10 or more drivers had General Speed-Related Issues curve-related errors versus those sequences where there were fewer than 10 is made. This shows that seven sequences fall Reordering the DPM sequences by the entry speed (which into the first category while five fall into the second. Inter- is defined as the lowest of the posted speed limit and the estingly, the curves where more drivers made errors were posted advisory) for the sequence and then examining the also higher-ranked in the curve hierarchy--that is, these number of drivers with curve-related errors revealed an inter- curves already had more extensive treatments. This was esting pattern. The two sequences (9 and 5) with the lowest especially true with the use of chevrons where four of the advisories (20 and 25 mph) had the largest numbers of drivers seven in the first category had chevrons present versus only (24 and 25) having errors. The four sequences (2, 3, 7, and 8) on one of the other group (of five). Contrarily, neither of the with 35-mph advisories were next with 19, 12, 8, and 7 drivers two worst curves had chevrons deployed. This leads to one having errors; and the 40- and 50-mph sequences (5a and 1) of two conclusions: either chevrons have little impact in had even lower numbers of drivers with errors (1 and 4). How- reducing drivers' curve-related errors or drivers would have ever, the sequences with the highest speed limits (55 mph) experienced even more errors if the chevrons had not been were mixed with three (6, 11, and 10) having a higher num- present. ber of errors (19, 18, and 13) and one (4) with only one driver Examining the initial speeds for both groups of curve making an error. Sequences 10 and 11 both consist of multi- sequences reveals that while the average drivers approached ple curves and are ranked "high" because in each sequence, the first category of curves a little faster than the other the initial curve has no advisory but there are advisories and (41 mph versus 39 mph), the MSP officers showed the additional treatments for a subsequent curve. Notwithstand- opposite (44 mph versus 46 mph). ing that Sequences 10 and 11 have advisories within the Interestingly, the two sequences on gravel roads showed sequence, the preponderance of drivers making errors come opposite results. For one sequence (4), there were very few at "both ends" of the speed scale--at curves that have very errors as drivers approached and drove through the curve con- low advisories and those where the initial speed is high. While servatively, negotiating them with ease. The other sequence some issues could be taken with the definition of an error or (6), which consisted of a much more abrupt curve with less